Hello ,
if vendor is not accepting this material then why u need it to be done with reference to PO.
Try the solution i have provided in my earlier reply
you can try somehting like this
1) while scrapping use any available text field to enter PO number(you can make it mandatory for MT)
2) create a report based on above data to track the scrap against POs.
only issue in this i see is you need to enter individual po scrap and cant make a collective scrap for same material
thanks
kUnal