diff -r 8684df73e037 shipment.py
--- a/trytond/trytond/modules/stock/shipment.py	Fri Sep 04 16:29:38 2015 +0200
+++ b/trytond/trytond/modules/stock/shipment.py	Fri Sep 04 16:30:32 2015 +0200
@@ -1111,6 +1111,14 @@
             unit_price=move.unit_price,
             )

+    @staticmethod
+    def _sync_inventory_to_outgoing_grouping_key(move, type):
+        '''
+        Returns the key used to sync inventory and outgoing moves for move
+        Type contains outgoing or incomming to indicate the kind of move
+        '''
+        return move.product.id
+
     @classmethod
     def _sync_inventory_to_outgoing(cls, shipments):
         'Synchronise outgoing moves with inventory moves'
@@ -1125,8 +1133,10 @@
                     continue
                 quantity = Uom.compute_qty(move.uom, move.quantity,
                         move.product.default_uom, round=False)
-                outgoing_qty.setdefault(move.product.id, 0.0)
-                outgoing_qty[move.product.id] += quantity
+                key = cls._sync_inventory_to_outgoing_grouping_key(move,
+                    'outgoing')
+                outgoing_qty.setdefault(key, 0.0)
+                outgoing_qty[key] += quantity

             to_create = []
             for move in shipment.inventory_moves:
@@ -1134,19 +1144,20 @@
                     continue
                 qty_default_uom = Uom.compute_qty(move.uom, move.quantity,
                     move.product.default_uom, round=False)
+                key = cls._sync_inventory_to_outgoing_grouping_key(move,
+                    'inventory')
                 # Check if the outgoing move doesn't exist already
-                if outgoing_qty.get(move.product.id):
+                if outgoing_qty.get(key):
                     # If it exist, decrease the sum
-                    if qty_default_uom <= outgoing_qty[move.product.id]:
-                        outgoing_qty[move.product.id] -= qty_default_uom
+                    if qty_default_uom <= outgoing_qty[key]:
+                        outgoing_qty[key] -= qty_default_uom
                         continue
                     # Else create the complement
                     else:
-                        out_quantity = (qty_default_uom
-                            - outgoing_qty[move.product.id])
+                        out_quantity = (qty_default_uom - outgoing_qty[key])
                         out_quantity = Uom.compute_qty(
                             move.product.default_uom, out_quantity, move.uom)
-                        outgoing_qty[move.product.id] = 0.0
+                        outgoing_qty[key] = 0.0
                 else:
                     out_quantity = move.quantity

@@ -1164,16 +1175,18 @@
             for move in shipment.outgoing_moves:
                 if move.state == 'cancel':
                     continue
-                if outgoing_qty.get(move.product.id, 0.0) > 0.0:
+                key = cls._sync_inventory_to_outgoing_grouping_key(move,
+                    'outgoing')
+                if outgoing_qty.get(key, 0.0) > 0.0:
                     exc_qty = Uom.compute_qty(move.product.default_uom,
-                            outgoing_qty[move.product.id], move.uom)
+                            outgoing_qty[key], move.uom)
                     removed_qty = Uom.compute_qty(move.uom,
                         min(exc_qty, move.quantity), move.product.default_uom,
                         round=False)
                     Move.write([move], {
                             'quantity': max(0.0, move.quantity - exc_qty),
                             })
-                    outgoing_qty[move.product.id] -= removed_qty
+                    outgoing_qty[key] -= removed_qty

     @classmethod
     @ModelView.button