diff --git a/pos/services/accounting_engine.py b/pos/services/accounting_engine.py
new file mode 100644
index 0000000..c556006
--- /dev/null
+++ b/pos/services/accounting_engine.py
@@ -0,0 +1,604 @@
+# /home/Autopartes/pos/services/accounting_engine.py
+"""Accounting engine: automatic journal entry generation for business operations.
+
+Every sale, purchase, cash cut, credit payment, and cancellation produces
+balanced journal entries (polizas). The engine looks up SAT account IDs
+by code and creates journal_entries + journal_entry_lines records.
+
+Account codes (from SAT seed):
+    110 = Caja
+    111 = Bancos
+    120 = Clientes
+    130 = Inventarios
+    140 = IVA Acreditable
+    210 = Proveedores
+    220 = IVA Trasladado
+    410 = Ventas
+    420 = Devoluciones sobre Ventas
+    510 = Costo de Mercancia Vendida
+
+All functions receive a psycopg2 connection (caller controls commit).
+"""
+
+from datetime import date
+from decimal import Decimal, ROUND_HALF_UP
+from flask import g
+
+
+def _to_dec(val):
+    """Convert a value to Decimal for precise arithmetic."""
+    if val is None:
+        return Decimal('0')
+    return Decimal(str(val))
+
+
+TWO = Decimal('0.01')
+
+
+def _get_account_id(cur, code):
+    """Look up account ID by code. Raises ValueError if not found."""
+    cur.execute("SELECT id FROM accounts WHERE code = %s AND is_active = true", (code,))
+    row = cur.fetchone()
+    if not row:
+        raise ValueError(f"Account with code '{code}' not found")
+    return row[0]
+
+
+def _get_account_ids(cur, codes):
+    """Look up multiple account IDs by code. Returns dict {code: id}."""
+    result = {}
+    for code in codes:
+        result[code] = _get_account_id(cur, code)
+    return result
+
+
+def get_next_entry_number(conn):
+    """Get the next sequential journal entry number.
+
+    Uses a simple MAX+1 approach. For high-concurrency environments this
+    could be replaced with a sequence, but for single-tenant refaccionarias
+    the transaction-level lock from the INSERT is sufficient.
+
+    Args:
+        conn: psycopg2 connection to tenant DB
+
+    Returns:
+        int: next entry number (starts at 1)
+    """
+    cur = conn.cursor()
+    cur.execute("SELECT COALESCE(MAX(entry_number), 0) + 1 FROM journal_entries")
+    number = cur.fetchone()[0]
+    cur.close()
+    return number
+
+
+def _check_period_open(cur, entry_date):
+    """Verify the fiscal period for the given date is open.
+
+    If no fiscal_periods row exists for the month, it is considered open
+    (periods are only created when explicitly closed).
+
+    Args:
+        cur: psycopg2 cursor
+        entry_date: date object
+
+    Raises:
+        ValueError: if the period is closed
+    """
+    cur.execute("""
+        SELECT status FROM fiscal_periods
+        WHERE year = %s AND month = %s
+    """, (entry_date.year, entry_date.month))
+    row = cur.fetchone()
+    if row and row[0] == 'closed':
+        raise ValueError(
+            f"Fiscal period {entry_date.year}-{entry_date.month:02d} is closed. "
+            f"Cannot create journal entries in a closed period."
+        )
+
+
+def _create_entry(cur, entry_number, entry_date, entry_type, description,
+                  reference_type, reference_id, lines, is_auto=True):
+    """Create a journal entry with its lines.
+
+    Validates that total debits == total credits before inserting.
+
+    Args:
+        cur: psycopg2 cursor
+        entry_number: int sequential number
+        entry_date: date
+        entry_type: 'ingreso' | 'egreso' | 'diario' | 'poliza'
+        description: str
+        reference_type: 'sale' | 'purchase' | 'cash_register' | 'payment' | None
+        reference_id: int or None
+        lines: list of dicts with keys: account_id, debit, credit, description
+        is_auto: bool (True for system-generated entries)
+
+    Returns:
+        int: journal entry ID
+
+    Raises:
+        ValueError: if debits != credits
+    """
+    # Validate balance
+    total_debit = sum(_to_dec(line['debit']) for line in lines)
+    total_credit = sum(_to_dec(line['credit']) for line in lines)
+
+    if total_debit.quantize(TWO, ROUND_HALF_UP) != total_credit.quantize(TWO, ROUND_HALF_UP):
+        raise ValueError(
+            f"Unbalanced entry: debits={total_debit} credits={total_credit}"
+        )
+
+    created_by = getattr(g, 'employee_id', None)
+
+    cur.execute("""
+        INSERT INTO journal_entries
+            (entry_number, date, type, description, reference_type, reference_id,
+             status, created_by, is_auto)
+        VALUES (%s, %s, %s, %s, %s, %s, 'posted', %s, %s)
+        RETURNING id
+    """, (
+        entry_number, entry_date, entry_type, description,
+        reference_type, reference_id, created_by, is_auto
+    ))
+    entry_id = cur.fetchone()[0]
+
+    for line in lines:
+        debit = float(_to_dec(line['debit']).quantize(TWO, ROUND_HALF_UP))
+        credit = float(_to_dec(line['credit']).quantize(TWO, ROUND_HALF_UP))
+        if debit == 0 and credit == 0:
+            continue  # skip zero lines
+        cur.execute("""
+            INSERT INTO journal_entry_lines
+                (journal_entry_id, account_id, debit, credit, description)
+            VALUES (%s, %s, %s, %s, %s)
+        """, (entry_id, line['account_id'], debit, credit, line.get('description', '')))
+
+    return entry_id
+
+
+def record_sale_entry(conn, sale):
+    """Generate journal entries for a completed sale.
+
+    For a cash sale:
+        Debit  110 Caja (or 111 Bancos for transferencia/tarjeta)  = total
+        Credit 410 Ventas                                          = subtotal
+        Credit 220 IVA Trasladado                                  = tax_total
+        Debit  510 Costo de Mercancia Vendida                      = sum(unit_cost * qty)
+        Credit 130 Inventarios                                     = sum(unit_cost * qty)
+
+    For a credit sale:
+        Debit  120 Clientes                                        = total
+        Credit 410 Ventas                                          = subtotal
+        Credit 220 IVA Trasladado                                  = tax_total
+        Debit  510 Costo de Mercancia Vendida                      = sum(unit_cost * qty)
+        Credit 130 Inventarios                                     = sum(unit_cost * qty)
+
+    Args:
+        conn: psycopg2 connection to tenant DB
+        sale: dict from process_sale() with keys:
+            id, sale_type, payment_method, subtotal, discount_total,
+            tax_total, total, items (with unit_cost, quantity)
+
+    Returns:
+        int: journal entry ID
+    """
+    cur = conn.cursor()
+    entry_date = date.today()
+    _check_period_open(cur, entry_date)
+
+    entry_number = get_next_entry_number(conn)
+    sale_id = sale['id']
+    sale_type = sale.get('sale_type', 'cash')
+    payment_method = sale.get('payment_method', 'efectivo')
+    subtotal = _to_dec(sale['subtotal'])
+    tax_total = _to_dec(sale['tax_total'])
+    total = _to_dec(sale['total'])
+
+    # Calculate total cost of goods sold
+    cost_total = Decimal('0')
+    for item in sale.get('items', []):
+        item_cost = _to_dec(item.get('unit_cost', 0)) * int(item.get('quantity', 1))
+        cost_total += item_cost.quantize(TWO, ROUND_HALF_UP)
+
+    # Determine debit account for payment received
+    accounts = _get_account_ids(cur, ['110', '111', '120', '130', '220', '410', '510'])
+
+    if sale_type == 'credit':
+        payment_account_id = accounts['120']  # Clientes
+        payment_desc = f'Clientes - Venta a credito #{sale_id}'
+    elif payment_method in ('transferencia', 'tarjeta'):
+        payment_account_id = accounts['111']  # Bancos
+        payment_desc = f'Bancos - Venta #{sale_id} ({payment_method})'
+    else:
+        payment_account_id = accounts['110']  # Caja
+        payment_desc = f'Caja - Venta #{sale_id} (efectivo)'
+
+    lines = [
+        # Payment received (debit)
+        {
+            'account_id': payment_account_id,
+            'debit': float(total.quantize(TWO, ROUND_HALF_UP)),
+            'credit': 0,
+            'description': payment_desc,
+        },
+        # Revenue (credit)
+        {
+            'account_id': accounts['410'],
+            'debit': 0,
+            'credit': float(subtotal.quantize(TWO, ROUND_HALF_UP)),
+            'description': f'Ventas - Venta #{sale_id}',
+        },
+        # IVA Trasladado (credit)
+        {
+            'account_id': accounts['220'],
+            'debit': 0,
+            'credit': float(tax_total.quantize(TWO, ROUND_HALF_UP)),
+            'description': f'IVA Trasladado - Venta #{sale_id}',
+        },
+    ]
+
+    # Cost of goods sold entries (only if cost > 0)
+    if cost_total > 0:
+        lines.append({
+            'account_id': accounts['510'],
+            'debit': float(cost_total.quantize(TWO, ROUND_HALF_UP)),
+            'credit': 0,
+            'description': f'Costo mercancia - Venta #{sale_id}',
+        })
+        lines.append({
+            'account_id': accounts['130'],
+            'debit': 0,
+            'credit': float(cost_total.quantize(TWO, ROUND_HALF_UP)),
+            'description': f'Inventarios - Venta #{sale_id}',
+        })
+
+    entry_id = _create_entry(
+        cur, entry_number, entry_date, 'ingreso',
+        f'Venta #{sale_id} - {payment_method}',
+        'sale', sale_id, lines
+    )
+
+    cur.close()
+    return entry_id
+
+
+def record_purchase_entry(conn, purchase_data):
+    """Generate journal entries for a purchase (inventory receipt).
+
+    Debit  130 Inventarios        = subtotal (cost of goods)
+    Debit  140 IVA Acreditable    = tax amount
+    Credit 210 Proveedores        = total (subtotal + tax)
+
+    Args:
+        conn: psycopg2 connection
+        purchase_data: dict with keys:
+            reference_id: int (purchase order or operation ID)
+            subtotal: float (cost of goods before tax)
+            tax_amount: float (IVA 16%)
+            total: float (subtotal + tax)
+            supplier_name: str (optional, for description)
+
+    Returns:
+        int: journal entry ID
+    """
+    cur = conn.cursor()
+    entry_date = date.today()
+    _check_period_open(cur, entry_date)
+
+    entry_number = get_next_entry_number(conn)
+    ref_id = purchase_data.get('reference_id')
+    subtotal = _to_dec(purchase_data['subtotal'])
+    tax_amount = _to_dec(purchase_data.get('tax_amount', 0))
+    total = _to_dec(purchase_data['total'])
+    supplier = purchase_data.get('supplier_name', 'Proveedor')
+
+    accounts = _get_account_ids(cur, ['130', '140', '210'])
+
+    lines = [
+        {
+            'account_id': accounts['130'],
+            'debit': float(subtotal.quantize(TWO, ROUND_HALF_UP)),
+            'credit': 0,
+            'description': f'Inventarios - Compra {supplier}',
+        },
+        {
+            'account_id': accounts['210'],
+            'debit': 0,
+            'credit': float(total.quantize(TWO, ROUND_HALF_UP)),
+            'description': f'Proveedores - Compra {supplier}',
+        },
+    ]
+
+    # IVA Acreditable (only if tax > 0)
+    if tax_amount > 0:
+        lines.append({
+            'account_id': accounts['140'],
+            'debit': float(tax_amount.quantize(TWO, ROUND_HALF_UP)),
+            'credit': 0,
+            'description': f'IVA Acreditable - Compra {supplier}',
+        })
+
+    entry_id = _create_entry(
+        cur, entry_number, entry_date, 'diario',
+        f'Compra - {supplier}',
+        'purchase', ref_id, lines
+    )
+
+    cur.close()
+    return entry_id
+
+
+def record_cash_cut_entry(conn, register):
+    """Generate journal entry for a cash register close (corte Z).
+
+    Moves cash from register (Caja) to bank (Bancos):
+        Debit  111 Bancos   = closing_amount
+        Credit 110 Caja     = closing_amount
+
+    Args:
+        conn: psycopg2 connection
+        register: dict with keys:
+            id: int (cash register ID)
+            closing_amount: float
+            register_number: int
+
+    Returns:
+        int: journal entry ID
+    """
+    cur = conn.cursor()
+    entry_date = date.today()
+    _check_period_open(cur, entry_date)
+
+    entry_number = get_next_entry_number(conn)
+    reg_id = register['id']
+    amount = _to_dec(register['closing_amount'])
+    reg_num = register.get('register_number', '?')
+
+    if amount <= 0:
+        cur.close()
+        return None  # No entry for zero/negative close
+
+    accounts = _get_account_ids(cur, ['110', '111'])
+
+    lines = [
+        {
+            'account_id': accounts['111'],
+            'debit': float(amount.quantize(TWO, ROUND_HALF_UP)),
+            'credit': 0,
+            'description': f'Bancos - Corte caja #{reg_num}',
+        },
+        {
+            'account_id': accounts['110'],
+            'debit': 0,
+            'credit': float(amount.quantize(TWO, ROUND_HALF_UP)),
+            'description': f'Caja - Corte caja #{reg_num}',
+        },
+    ]
+
+    entry_id = _create_entry(
+        cur, entry_number, entry_date, 'diario',
+        f'Corte de caja #{reg_num} (registro #{reg_id})',
+        'cash_register', reg_id, lines
+    )
+
+    cur.close()
+    return entry_id
+
+
+def record_credit_payment_entry(conn, payment):
+    """Generate journal entry for a customer credit payment.
+
+    Debit  111 Bancos (or 110 Caja)  = amount
+    Credit 120 Clientes              = amount
+
+    Args:
+        conn: psycopg2 connection
+        payment: dict with keys:
+            customer_id: int
+            customer_name: str (optional)
+            amount: float
+            payment_method: 'efectivo' | 'transferencia' | 'tarjeta'
+            reference_id: int (optional, e.g. sale_id being paid)
+
+    Returns:
+        int: journal entry ID
+    """
+    cur = conn.cursor()
+    entry_date = date.today()
+    _check_period_open(cur, entry_date)
+
+    entry_number = get_next_entry_number(conn)
+    amount = _to_dec(payment['amount'])
+    customer_name = payment.get('customer_name', f"Cliente #{payment['customer_id']}")
+    method = payment.get('payment_method', 'efectivo')
+    ref_id = payment.get('reference_id')
+
+    accounts = _get_account_ids(cur, ['110', '111', '120'])
+
+    if method in ('transferencia', 'tarjeta'):
+        debit_account = accounts['111']  # Bancos
+        debit_desc = f'Bancos - Cobro credito {customer_name}'
+    else:
+        debit_account = accounts['110']  # Caja
+        debit_desc = f'Caja - Cobro credito {customer_name}'
+
+    lines = [
+        {
+            'account_id': debit_account,
+            'debit': float(amount.quantize(TWO, ROUND_HALF_UP)),
+            'credit': 0,
+            'description': debit_desc,
+        },
+        {
+            'account_id': accounts['120'],
+            'debit': 0,
+            'credit': float(amount.quantize(TWO, ROUND_HALF_UP)),
+            'description': f'Clientes - Cobro credito {customer_name}',
+        },
+    ]
+
+    entry_id = _create_entry(
+        cur, entry_number, entry_date, 'ingreso',
+        f'Cobro credito - {customer_name} ({method})',
+        'payment', ref_id, lines
+    )
+
+    cur.close()
+    return entry_id
+
+
+def record_cancellation_entry(conn, sale):
+    """Generate reverse journal entry for a cancelled sale.
+
+    This is the exact reverse of record_sale_entry():
+        Credit payment account (Caja/Bancos/Clientes) = total
+        Debit  410 Ventas                              = subtotal
+        Debit  220 IVA Trasladado                      = tax_total
+        Credit 510 Costo de Mercancia Vendida          = cost_total
+        Debit  130 Inventarios                         = cost_total
+
+    Args:
+        conn: psycopg2 connection
+        sale: dict with the same structure as record_sale_entry() expects.
+            If items are not present, looks up from DB.
+
+    Returns:
+        int: journal entry ID
+    """
+    cur = conn.cursor()
+    entry_date = date.today()
+    _check_period_open(cur, entry_date)
+
+    entry_number = get_next_entry_number(conn)
+    sale_id = sale['id']
+    sale_type = sale.get('sale_type', 'cash')
+    payment_method = sale.get('payment_method', 'efectivo')
+    subtotal = _to_dec(sale.get('subtotal', 0))
+    tax_total = _to_dec(sale.get('tax_total', 0))
+    total = _to_dec(sale.get('total', 0))
+
+    # If sale dict lacks items, look up from DB
+    items = sale.get('items')
+    if not items:
+        cur.execute("""
+            SELECT unit_cost, quantity FROM sale_items WHERE sale_id = %s
+        """, (sale_id,))
+        items = [{'unit_cost': float(r[0]) if r[0] else 0, 'quantity': r[1]}
+                 for r in cur.fetchall()]
+
+    # If sale dict lacks totals, look up from DB
+    if total == 0:
+        cur.execute("""
+            SELECT subtotal, tax_total, total, sale_type, payment_method
+            FROM sales WHERE id = %s
+        """, (sale_id,))
+        row = cur.fetchone()
+        if row:
+            subtotal = _to_dec(row[0])
+            tax_total = _to_dec(row[1])
+            total = _to_dec(row[2])
+            sale_type = row[3]
+            payment_method = row[4]
+
+    cost_total = Decimal('0')
+    for item in items:
+        item_cost = _to_dec(item.get('unit_cost', 0)) * int(item.get('quantity', 1))
+        cost_total += item_cost.quantize(TWO, ROUND_HALF_UP)
+
+    accounts = _get_account_ids(cur, ['110', '111', '120', '130', '220', '410', '510'])
+
+    # Determine which payment account to credit (reverse of debit in sale)
+    if sale_type == 'credit':
+        payment_account_id = accounts['120']
+        payment_desc = f'Clientes - Cancelacion venta #{sale_id}'
+    elif payment_method in ('transferencia', 'tarjeta'):
+        payment_account_id = accounts['111']
+        payment_desc = f'Bancos - Cancelacion venta #{sale_id}'
+    else:
+        payment_account_id = accounts['110']
+        payment_desc = f'Caja - Cancelacion venta #{sale_id}'
+
+    lines = [
+        # Reverse payment (credit the account that was debited)
+        {
+            'account_id': payment_account_id,
+            'debit': 0,
+            'credit': float(total.quantize(TWO, ROUND_HALF_UP)),
+            'description': payment_desc,
+        },
+        # Reverse revenue (debit Ventas)
+        {
+            'account_id': accounts['410'],
+            'debit': float(subtotal.quantize(TWO, ROUND_HALF_UP)),
+            'credit': 0,
+            'description': f'Ventas - Cancelacion venta #{sale_id}',
+        },
+        # Reverse IVA (debit IVA Trasladado)
+        {
+            'account_id': accounts['220'],
+            'debit': float(tax_total.quantize(TWO, ROUND_HALF_UP)),
+            'credit': 0,
+            'description': f'IVA Trasladado - Cancelacion venta #{sale_id}',
+        },
+    ]
+
+    # Reverse COGS entries (only if cost > 0)
+    if cost_total > 0:
+        lines.append({
+            'account_id': accounts['510'],
+            'debit': 0,
+            'credit': float(cost_total.quantize(TWO, ROUND_HALF_UP)),
+            'description': f'Costo mercancia - Cancelacion venta #{sale_id}',
+        })
+        lines.append({
+            'account_id': accounts['130'],
+            'debit': float(cost_total.quantize(TWO, ROUND_HALF_UP)),
+            'credit': 0,
+            'description': f'Inventarios - Cancelacion venta #{sale_id}',
+        })
+
+    entry_id = _create_entry(
+        cur, entry_number, entry_date, 'egreso',
+        f'Cancelacion venta #{sale_id}',
+        'sale', sale_id, lines
+    )
+
+    cur.close()
+    return entry_id
+
+
+def create_manual_entry(conn, entry_data):
+    """Create a manual journal entry (type='diario').
+
+    Used by accountants for adjustments not tied to a specific operation.
+
+    Args:
+        conn: psycopg2 connection
+        entry_data: dict with keys:
+            date: str 'YYYY-MM-DD'
+            description: str
+            lines: [{account_id, debit, credit, description}]
+
+    Returns:
+        int: journal entry ID
+
+    Raises:
+        ValueError: if entry is unbalanced or period is closed
+    """
+    cur = conn.cursor()
+    entry_date = date.fromisoformat(entry_data['date'])
+    _check_period_open(cur, entry_date)
+
+    entry_number = get_next_entry_number(conn)
+
+    entry_id = _create_entry(
+        cur, entry_number, entry_date, 'diario',
+        entry_data.get('description', 'Poliza manual'),
+        None, None,
+        entry_data['lines'],
+        is_auto=False
+    )
+
+    cur.close()
+    return entry_id