[IMP] l10n_ar_tax: revert withholding rounding patch and improve calculation

Revertimos el cambio de este pull request #1218 y mejoramos cálculo de retención tal como lo hizo odoo aquí odoo/odoo@ac521b2#diff-dab181eb3722c2603ab3586daa152293cfa7b98bdccc7bd7508eec8062de67d9R71
Ticket: 101778

Author: pablohmontenegro

l10n_ar_tax/models/l10n_ar_payment_withholding.py

@@ -103,11 +103,9 @@ def _tax_compute_all_helper(self):
             product=False,
             partner=False,
             is_refund=False,
+            rounding_method="round_per_line",
         )
-        tax_amount = self.currency_id.round(taxes_res["total_included"] - taxes_res["total_excluded"])
-        # TODO: When Odoo fixes the compute_all method of account_tax, uncomment the line below and
-        # remove the line above. See Adhoc ticket 101778 for more information.
-        # tax_amount = taxes_res["taxes"][0]["amount"]
+        tax_amount = taxes_res["taxes"][0]["amount"]
         tax_account_id = taxes_res["taxes"][0]["account_id"]
         tax_repartition_line_id = taxes_res["taxes"][0]["tax_repartition_line_id"]
 

l10n_ar_tax/tests/test_payment_receiptbook_and_withholding.py

@@ -371,3 +371,124 @@ def test_create_vendor_payment_with_receiptbook_and_withholdings(self):
                 {"debit": 655000.0, "credit": 0.0, "amount_currency": 655000.0},
             ],
         )
+
+    def test_withholding_amounts(self):
+        """Verify withholding amount precision under 'round_globally' rounding method.
+
+        With price_unit=391683 and a 4.5% withholding tax, the exact amount is
+        391683 * 0.045 = 17625.735. Under 'round_globally', this rounds to 17625.74.
+        The test ensures the rounding method is respected and the resulting
+        withholding line carries the correctly rounded amount_currency.
+        """
+        company = self.env.company
+        previous_rounding_method = company.tax_calculation_rounding_method
+        company.tax_calculation_rounding_method = "round_globally"
+        try:
+            # Create a vendor bill with a single line subject to 21% VAT
+            in_invoice_wht = self.env["account.move"].create(
+                {
+                    "move_type": "in_invoice",
+                    "date": "2025-10-01",
+                    "invoice_date": "2025-10-01",
+                    "partner_id": self.env.ref("l10n_ar_tax.res_partner_adhoc_caba").id,
+                    "invoice_line_ids": [
+                        Command.create(
+                            {
+                                "product_id": self.product_a.id,
+                                "price_unit": 391683,
+                                "tax_ids": [Command.set(self.tax_21.ids)],
+                            }
+                        )
+                    ],
+                    "l10n_latam_document_number": "2-5",
+                }
+            )
+            in_invoice_wht.action_post()
+
+            # Set withholding tax rate to 4.5% (produces a non-trivial rounding case)
+            self.tax_wth_test_1.write({"amount": 4.5})
+
+            # Create fiscal position with withholding for CABA partner
+            fiscal_pos = self.env["account.fiscal.position"].create(
+                {
+                    "name": "IIBB CABA Rounding",
+                    "l10n_ar_afip_responsibility_type_ids": [(6, 0, [self.env.ref("l10n_ar.res_IVARI").id])],
+                    "sequence": 10,
+                    "auto_apply": True,
+                    "country_id": self.env.ref("base.ar").id,
+                    "company_id": self.company_ri.id,
+                    "state_ids": [(6, 0, [self.env.ref("base.state_ar_c").id])],
+                }
+            )
+            self.env["account.fiscal.position.l10n_ar_tax"].create(
+                {
+                    "fiscal_position_id": fiscal_pos.id,
+                    "default_tax_id": self.tax_wth_test_1.id,
+                    "tax_type": "withholding",
+                }
+            )
+
+            # Create payment using register payment context
+            action_context = in_invoice_wht.action_register_payment()["context"]
+            vals = {
+                "journal_id": self.company_bank_journal.id,
+                "amount": in_invoice_wht.amount_total,
+                "date": "2025-10-01",
+            }
+            payment = self.env["account.payment"].with_context(**action_context).create(vals)
+
+            # In direct payment creation flow, amount must be net of withholdings to fully reconcile the invoice.
+            payment.amount = in_invoice_wht.amount_total - payment.withholdings_amount
+            payment.action_post()
+
+            self.assertEqual(payment.company_id.tax_calculation_rounding_method, "round_globally")
+            # 391683 * 21% = 82253.43 (VAT) -> total invoice: 473936.43
+            # 391683 * 4.5% = 17625.735 -> rounded globally to 17625.74 (withholding)
+            # net payment (liquidity): 473936.43 - 17625.74 = 456310.69
+            self.assertRecordValues(
+                payment.move_id.line_ids.sorted("balance"),
+                [
+                    # Liquidity line:
+                    {
+                        "debit": 0.0,
+                        "credit": 456310.69,
+                        "currency_id": payment.currency_id.id,
+                        "amount_currency": -456310.69,
+                        "reconciled": False,
+                    },
+                    # base line:
+                    {
+                        "debit": 0.0,
+                        "credit": 391683.0,
+                        "currency_id": payment.currency_id.id,
+                        "amount_currency": -391683.0,
+                        "reconciled": False,
+                    },
+                    # withholding line:
+                    {
+                        "debit": 0.0,
+                        "credit": 17625.74,
+                        "currency_id": payment.currency_id.id,
+                        "amount_currency": -17625.74,
+                        "reconciled": False,
+                    },
+                    # base line:
+                    {
+                        "debit": 391683.0,
+                        "credit": 0.0,
+                        "currency_id": payment.currency_id.id,
+                        "amount_currency": 391683.0,
+                        "reconciled": False,
+                    },
+                    # Receivable line:
+                    {
+                        "debit": 473936.43,
+                        "credit": 0.0,
+                        "currency_id": payment.currency_id.id,
+                        "amount_currency": 473936.43,
+                        "reconciled": True,
+                    },
+                ],
+            )
+        finally:
+            company.tax_calculation_rounding_method = previous_rounding_method