[REF] stock_account: unit price of rereturn

`_get_price_unit` is used to get the price to value a stock move and
returns `unit_price` if present (most of the times it is set by purchase_stock
to copy the price unit of po line) or the standard price.

Note that a return copies the original move and `price_unit` is set as
copy False.

Before this patch, a return of a return used the standard price (due to
the copy False), which doens't bring any issue but could be confusing
as explained in the following receipt scenario.

On a receipt, return and rereturn. The original and rereturn move may
not have the same price if the standard price has changed between the
operations, which is weird since there's a unit price on a potentially
linked po line.

On an delivery, there won't be any effect since the valuation methods do
not use the result of `_get_price_unit` but force it to the standard
price (standard and avco) or run fifo.

task-1875873

addons/stock_account/models/stock_move.py

@@ -32,8 +32,13 @@ class StockMove(models.Model):
         return action_data
 
     def _get_price_unit(self):
-        """ Returns the unit price to store on the quant """
-        return not self.company_id.currency_id.is_zero(self.price_unit) and self.price_unit or self.product_id.standard_price
+        """ Returns the unit price to value this stock move """
+        self.ensure_one()
+        price_unit = self.price_unit
+        # If the move is a return, use the original move's price unit.
+        if self.origin_returned_move_id:
+            price_unit = self.origin_returned_move_id.price_unit
+        return not self.company_id.currency_id.is_zero(price_unit) and price_unit or self.product_id.standard_price
 
     @api.model
     def _get_in_base_domain(self, company_id=False):