A link from Production Order to Sales Order

Hi,
The following is the scenario. We have done a customization such that the 'Quantity' field in the Finished production order header updates the 'qty. to ship' in the sales order.so that that we could exactly ship only those items converted as finished production order.
But in some cases the production department produces 50% of the qty only and converts the released production order to finished besides the warning given by the system.
Now I should be able to update the rest of the 50% quantities to the back sales order so that the rest of the quantities can be shipped as a next consignment.But I dont have a production order in hand as it had been already converted to a finished production order. can anyone suggest any ideas as how this can be done.Thanks in advance.
Regards,Anantha