Use fixed location type to compute quantity to invoice for sale and purchase

Using the computed location to get the expected type is not reliable because
for return order, the location can be of different type and the stock move can
change of location type.

Closes #12203

modules/purchase/purchase.py

@@ -1735,15 +1735,13 @@ class Line(sequence_ordered(), ModelSQL, ModelView):
                 qty = Uom.compute_qty(move.uom, move.quantity, self.unit)
                 # Test only against from_location
                 # as it is what matters for purchase
-                src_type = self.from_location.type
+                src_type = 'supplier'
                 if (move.from_location.type == src_type
                         and move.to_location.type != src_type):
                     quantity += qty
                 elif (move.to_location.type == src_type
                         and move.from_location.type != src_type):
                     quantity -= qty
-            if self.quantity < 0:
-                quantity *= -1
             return quantity
 
     def _get_invoiced_quantity(self):