You could use PoS and each time a scrap metal is bought you could have product qty as (-ve) this generates a refund and inventory adjust Partner/ customer to WH/Stock is created.

The advantage is that you could do multiple transactions in a session while posted as consolidated. Inventory is also updated on session close without the need to validate individual receipts.

Alternately, create a blanket PO, for product (scrap metal) and individual receipts tracked against it. Once each receipt is validated, create backorder and on the PO create bill for the received qty.

Hi,

It's not a good idea to purchase scrap from the stores using POS. It's better to use the Odoo purchase module. With this module, when an employee reaches the store, they can create a purchase order for the product and then validate the receipt and bill. One of the benefits of using this method is that we can manage stock in Odoo as well.

ThankHi,

Of course, you can create the POS and then validate it when you get the payment. 

I can assume your walk-in clients do not need an invoice so you can stop your process there.