diff --git a/addons/purchase/stock.py b/addons/purchase/stock.py
index e6d0cb8fa6f..5f569e1964d 100644
--- a/addons/purchase/stock.py
+++ b/addons/purchase/stock.py
@@ -31,6 +31,13 @@ class stock_move(osv.osv):
             readonly=True),
     }
 
+    def get_price_unit(self, cr, uid, move, context=None):
+        """ Returns the unit price to store on the quant """
+        if move.purchase_line_id:
+            return move.price_unit
+
+        return super(stock_move, self).get_price_unit(cr, uid, move, context=context)
+
     def write(self, cr, uid, ids, vals, context=None):
         if isinstance(ids, (int, long)):
             ids = [ids]
@@ -117,6 +124,13 @@ class stock_move(osv.osv):
         """
             Attribute price to move, important in inter-company moves or receipts with only one partner
         """
+        # The method attribute_price of the parent class sets the price to the standard product
+        # price if move.price_unit is zero. We don't want this behavior in the case of a purchase
+        # order since we can purchase goods which are free of charge (e.g. 5 units offered if 100
+        # are purchased).
+        if move.purchase_line_id:
+            return
+
         code = self.get_code_from_locs(cr, uid, move, context=context)
         if not move.purchase_line_id and code == 'incoming' and not move.price_unit:
             partner = move.picking_id and move.picking_id.partner_id or False