[FIX] stock_account: base reversed anglo-saxon AML on original one

When reversing an invoice that contains some anglo-saxo AML, the new
anglo-saxo AML should have the same unit price than the original one

To reproduce the issue:
(Need account_accountant,purchase)
1. Create a product category PC:
    - Costing Method: AVCO
    - Inventory Valuation: Automated
2. Create a product P:
    - Type: Storable
    - Product Category: PC
3. Create a purchase order PO01 with 2 x P for $10
4. Confirm PO01 and process the receipt
5. Create and Post an invoice INV01 with 2 x P
    - As listed in the journal items, the anglo-saxo lines have been
generated with a value of $20
6. Create a purchase order PO02 with 2 x P for $20
    - The cost of P becomes $15
7. Confirm PO02 and process the receipt
8. Add a credit note to INV01:
    - Credit Method: Partial Refund
9. Set the quantity of P to 1
10. Post the new invoice INV02

Error: The anglo-saxo lines are listed in the journal items, which is
correct, but their value is $15 (the new cost of P) while it should be
$10

When reversing an invoice with the "partial refund" method, the
anglo-saxo lines are generated via the regular flow (as if it were a
"standard" invoice). Therefore, when getting the unit price of the line,
it uses the product's standard price. In such situation, it should use
the unit price of the original line.

OPW-2646926
OPW-2628215

Part-of: odoo/odoo#82547

addons/stock_account/models/account_move.py

@@ -233,4 +233,8 @@ class AccountMoveLine(models.Model):
         self.ensure_one()
         if not self.product_id:
             return self.price_unit
-        return self.product_id.with_company(self.company_id)._stock_account_get_anglo_saxon_price_unit(uom=self.product_uom_id)
+        original_line = self.move_id.reversed_entry_id.line_ids.filtered(lambda l: l.is_anglo_saxon_line
+            and l.product_id == self.product_id and l.product_uom_id == self.product_uom_id and l.price_unit >= 0)
+        original_line = original_line and original_line[0]
+        return original_line.price_unit if original_line \
+            else self.product_id.with_company(self.company_id)._stock_account_get_anglo_saxon_price_unit(uom=self.product_uom_id)

addons/stock_account/tests/test_stockvaluationlayer.py

@@ -898,3 +898,60 @@ class TestStockValuationChangeValuation(TestStockValuationCommon):
         self.assertEqual(len(self.product1.stock_valuation_layer_ids.mapped('account_move_id')), 2)
         self.assertEqual(len(self.product1.stock_valuation_layer_ids), 3)
 
+@tagged('post_install', '-at_install')
+class TestAngloSaxonAccounting(TestStockValuationCommon):
+    @classmethod
+    def setUpClass(cls):
+        super(TestAngloSaxonAccounting, cls).setUpClass()
+        cls.env.company.anglo_saxon_accounting = True
+        cls.stock_input_account, cls.stock_output_account, cls.stock_valuation_account, cls.expense_account, cls.stock_journal = _create_accounting_data(cls.env)
+        cls.product1.write({
+            'property_account_expense_id': cls.expense_account.id,
+        })
+        cls.product1.categ_id.write({
+            'property_valuation': 'real_time',
+            'property_stock_account_input_categ_id': cls.stock_input_account.id,
+            'property_stock_account_output_categ_id': cls.stock_output_account.id,
+            'property_stock_valuation_account_id': cls.stock_valuation_account.id,
+            'property_stock_journal': cls.stock_journal.id,
+        })
+        cls.default_journal_purchase =  cls.env['account.journal'].search([
+            ('company_id', '=', cls.env.company.id),
+            ('type', '=', 'purchase')
+        ], limit=1)
+
+    def test_avco_and_credit_note(self):
+        """
+        When reversing an invoice that contains some anglo-saxo AML, the new anglo-saxo AML should have the same value
+        """
+        self.product1.categ_id.property_cost_method = 'average'
+
+        self._make_in_move(self.product1, 2, unit_cost=10)
+
+        invoice_form = Form(self.env['account.move'].with_context(default_move_type='out_invoice'))
+        invoice_form.partner_id = self.env['res.partner'].create({'name': 'Super Client'})
+        with invoice_form.invoice_line_ids.new() as invoice_line_form:
+            invoice_line_form.product_id = self.product1
+            invoice_line_form.quantity = 2
+            invoice_line_form.price_unit = 25
+            invoice_line_form.account_id = self.default_journal_purchase.default_account_id
+        invoice = invoice_form.save()
+        invoice.action_post()
+
+        self._make_in_move(self.product1, 2, unit_cost=20)
+        self.assertEqual(self.product1.standard_price, 15)
+
+        refund_wizard = self.env['account.move.reversal'].with_context(active_model="account.move", active_ids=invoice.ids).create({
+            'refund_method': 'refund',
+        })
+        action = refund_wizard.reverse_moves()
+        reverse_invoice = self.env['account.move'].browse(action['res_id'])
+        with Form(reverse_invoice) as reverse_invoice_form:
+            with reverse_invoice_form.invoice_line_ids.edit(0) as line:
+                line.quantity = 1
+        reverse_invoice.action_post()
+
+        anglo_lines = reverse_invoice.line_ids.filtered(lambda l: l.is_anglo_saxon_line)
+        self.assertEqual(len(anglo_lines), 2)
+        self.assertEqual(abs(anglo_lines[0].balance), 10)
+        self.assertEqual(abs(anglo_lines[1].balance), 10)