diff --git a/addons/purchase/purchase.py b/addons/purchase/purchase.py
index 98e17b136f3..d3fdb96d35a 100644
--- a/addons/purchase/purchase.py
+++ b/addons/purchase/purchase.py
@@ -1634,26 +1634,18 @@ class procurement_order(osv.osv):
             for proc in procurements:
                 if po_prod_dict.get(proc.product_id.id):
                     po_line = po_prod_dict[proc.product_id.id]
-                    # FIXME: compute quantity using `_calc_new_qty_price` method.
-                    # new_qty, new_price = self._calc_new_qty_price(cr, uid, proc, po_line=po_line, context=context)
-                    uom_id = po_line.product_uom  # Convert to UoM of existing line
-                    qty = uom_obj._compute_qty_obj(cr, uid, proc.product_uom, proc.product_qty, uom_id)
-
                     if lines_to_update.get(po_line):
-                        lines_to_update[po_line] += [(proc.id, qty)]
+                        lines_to_update[po_line] += [proc]
                     else:
-                        lines_to_update[po_line] = [(proc.id, qty)]
+                        lines_to_update[po_line] = [proc]
                 else:
                     procs_to_create.append(proc)
 
-            procs = []
             # Update the quantities of the lines that need to
             for line in lines_to_update.keys():
-                tot_qty = 0
-                for proc, qty in lines_to_update[line]:
-                    tot_qty += qty
-                    self.message_post(cr, uid, proc, body=_("Quantity added in existing Purchase Order Line"), context=context)
-                line_values += [(1, line.id, {'product_qty': line.product_qty + tot_qty, 'procurement_ids': [(4, x[0]) for x in lines_to_update[line]]})]
+                for proc in lines_to_update[line]:
+                    self.message_post(cr, uid, proc.id, body=_("Quantity added in existing Purchase Order Line"), context=context)
+                line_values += [(1, line.id, {'procurement_ids': [(4, x[0].id) for x in lines_to_update[line]]})]
 
             # Create lines for which no line exists yet
             if procs_to_create:
@@ -1665,6 +1657,21 @@ class procurement_order(osv.osv):
                     self.message_post(cr, uid, [proc.id], body=_("Purchase line created and linked to an existing Purchase Order"), context=context)
             po_obj.write(cr, uid, [add_purchase], {'order_line': line_values},context=context)
 
+            # FIXME: partial fix of forward-port fe92dea. Ideally, we should use the method
+            # '_calc_new_qty_price'. However, this method was written for v8, where the PO line
+            # update was sequential, not in batch as it is now.
+            # DO NOT FORWARD PORT
+            for line, new_procs in lines_to_update.iteritems():
+                qty = 0.0
+                for proc in line.procurement_ids:
+                    if proc in new_procs or proc.state == 'running':
+                        qty += uom_obj._compute_qty(cr, uid,
+                            proc.product_uom.id, proc.product_qty, proc.product_id.uom_id.id, round=False)
+
+                new_qty = uom_obj._compute_qty(cr, uid, line.product_id.uom_id.id, qty, line.product_uom.id)
+                if new_qty > line.product_qty:
+                    line.write({'product_qty': new_qty})
+
             for procurement in procurements:
                 self.update_origin_po(cr, uid, po, procurement, context=context)