Segregate or flag the new open PO shipment qty created by Return to Vendor

In Oracle Purchasing, I have been searching for a way to segregate the open PO line shipment quantities that are created when a Return to Vendor (RTV) transaction is processed.  Currently, the PO line shpment quantity that is associated with the receipt number that is referenced on the RTV is incremented by the returned quantity.  This causes a problem for us because we try to measure the vendors' on time delivery performance based upon their deliver date vs the promise date on the PO line shipment.  The promise date for the replacement units ends up as the same as the original shipment. As