fix: prevent double counting of deduction lines by filtering write-off values in account payment move generation

models/account_payment.py

@@ -243,12 +243,23 @@ class AccountPayment(models.Model):
         - Bank: credit 1850 (final_payment_amount)
         Total: debit 2000 = credit 2000 (balanced)
         """
+        # Filter out existing deductions from write_off_line_vals to prevent double counting
+        # by Odoo's native balancing logic.
+        if write_off_line_vals and self.deduction_line_ids:
+            deduction_account_ids = self.deduction_line_ids.mapped('substract_account_id').ids
+            filtered_write_offs = []
+            for w in write_off_line_vals:
+                if w.get('account_id') not in deduction_account_ids:
+                    filtered_write_offs.append(w)
+            write_off_line_vals = filtered_write_offs
+
         # Get standard line values from parent
         line_vals_list = super()._prepare_move_line_default_vals(write_off_line_vals, force_balance)
         
         # Only modify if we have deductions
         if self.amount_substract and self.amount_substract > 0 and self.deduction_line_ids:
             if self.payment_type == 'outbound':
+
                 # Get existing deduction account IDs to prevent duplicates
                 existing_deduction_accounts = {
                     line.get('account_id')