diff -r 8ba590e21c94 purchase.py
--- a/trytond/trytond/modules/purchase/purchase.py	Tue Oct 06 10:09:51 2015 +0200
+++ b/trytond/trytond/modules/purchase/purchase.py	Tue Oct 06 10:16:28 2015 +0200
@@ -1048,7 +1048,6 @@
         return 'line'

     def get_move_done(self, name):
-        Uom = Pool().get('product.uom')
         done = True
         if not self.product:
             return True
@@ -1062,7 +1061,7 @@
                     and move.id not in skip_ids:
                 done = False
                 break
-            quantity -= Uom.compute_qty(move.uom, move.quantity, self.unit)
+            quantity -= move.origin_quantity
         if done:
             if quantity > 0.0:
                 done = False
@@ -1281,8 +1280,7 @@
             quantity = 0.0
             for move in self.moves:
                 if move.state == 'done':
-                    quantity += Uom.compute_qty(move.uom, move.quantity,
-                        self.unit)
+                    quantity += move.origin_quantity
                     if move.invoiced_quantity < move.quantity:
                         stock_moves.append(move)
         invoice_line.stock_moves = stock_moves
@@ -1293,8 +1291,7 @@
             if old_invoice_line.type != 'line':
                 continue
             if old_invoice_line.id not in skip_ids:
-                quantity -= Uom.compute_qty(old_invoice_line.unit,
-                        old_invoice_line.quantity, self.unit)
+                quantity -= old_invoice_line.origin_quantity

         rounding = self.unit.rounding if self.unit else 0.01
         invoice_line.quantity = Uom.round(quantity, rounding)
@@ -1352,8 +1349,7 @@
         quantity = abs(self.quantity)
         for move in self.moves:
             if move not in skip:
-                quantity -= Uom.compute_qty(move.uom, move.quantity,
-                    self.unit)
+                quantity -= move.origin_quantity

         quantity = Uom.round(quantity, self.unit.rounding)
         if quantity <= 0: