[FIX] sale_stock: invoice policy on delivered quantities

The created accounting entries have their debit/credit columns switched
because
  - they use the sign of the value to post to decide if it should be
    credit or debit
  - an out move always have a negative sign

As this code is always called on out moves, we can safely reverse the
sign when getting the price_unit of the move.

Fixes #20080

addons/sale_stock/models/account_invoice.py

@@ -51,7 +51,7 @@ def _compute_average_price(self, qty_done, quantity, moves):
                 qty_to_consider = invoiced_qty - qty_done
             qty_to_consider = min(qty_to_consider, quantity - qty_delivered)
             qty_delivered += qty_to_consider
-            average_price_unit = (average_price_unit * (qty_delivered - qty_to_consider) + move.price_unit * qty_to_consider) / qty_delivered
+            average_price_unit = (average_price_unit * (qty_delivered - qty_to_consider) + (-1 * move.price_unit) * qty_to_consider) / qty_delivered
             if qty_delivered == quantity:
                 break
         return average_price_unit