Hi,
Hope the scrap has to be valuated with zero price. If so, I would suggest to create a free of cost PO and then receive the item. Now, you may create a new customer master for the same vendor and link in the customer master - control data tab. For receiving the payment from vendor, you can post direct FI postings with FB70 for the corresponding customer. Test the scenario and revert back.
Regards,
AKPT