[FIX] stock_account: correct value for returns

Before this commit:
When returning a transfer in AVCO/FIFO, the return's value is not always
the same as the original delivery. The unit cost for the return was
rounded, which introduced rounding errors.

After this commit:
When returning a transfer in AVCO/FIFO, the return's unit cost is not
rounded, allowing the correct value to be set.

Steps to reproduce:
1. Create a product and change its category costing method to AVCO
2. Purchase the product qty: 1 price: 13.13
3. Purchase the product qty: 1 price: 12.20
4. Sell the 2 product and deliver it
5. Create a return for the delivery
6. The value on the delivery is 25.33, but 25.34 on the return

opw-3358531

closes odoo/odoo#126546

Signed-off-by: William Henrotin (whe) <whe@odoo.com>

addons/stock_account/models/stock_move.py

@@ -46,7 +46,7 @@ class StockMove(models.Model):
                 layers = layers.filtered(lambda l: float_compare(l.value, 0, precision_rounding=l.product_id.uom_id.rounding) <= 0)
             layers |= layers.stock_valuation_layer_ids
             quantity = sum(layers.mapped("quantity"))
-            return layers.currency_id.round(sum(layers.mapped("value")) / quantity) if not float_is_zero(quantity, precision_rounding=layers.uom_id.rounding) else 0
+            return sum(layers.mapped("value")) / quantity if not float_is_zero(quantity, precision_rounding=layers.uom_id.rounding) else 0
         return price_unit if not float_is_zero(price_unit, precision) or self._should_force_price_unit() else self.product_id.standard_price
 
     @api.model

addons/stock_account/tests/test_stockvaluationlayer.py

@@ -580,6 +580,18 @@ class TestStockValuationAVCO(TestStockValuationCommon):
         self.assertEqual(self.product1.quantity_svl, 10)
         self.assertEqual(self.product1.standard_price, 2)
 
+    def test_return_delivery_rounding(self):
+        self.product1.product_tmpl_id.categ_id.property_valuation = 'manual_periodic'
+        self.product1.write({"standard_price": 1})
+        self._make_in_move(self.product1, 1, unit_cost=13.13)
+        self._make_in_move(self.product1, 1, unit_cost=12.20)
+        move3 = self._make_out_move(self.product1, 2, create_picking=True)
+        move4 = self._make_return(move3, 2)
+
+        self.assertAlmostEqual(abs(move3.stock_valuation_layer_ids[0].value), abs(move4.stock_valuation_layer_ids[0].value))
+        self.assertAlmostEqual(self.product1.value_svl, 25.33)
+        self.assertEqual(self.product1.quantity_svl, 2)
+
 
 class TestStockValuationFIFO(TestStockValuationCommon):
     def setUp(self):