RTV price variance of original item purchase vs return purchase

Summary:

Hi all, I am looking for some guidance on the direct / indirect material RTV process where the amount back from the supplier, is less than the original purchase.

Scenario 1: Sales order is raised for a year long customer project and they order 1000 units of cameras for a project installations. The cameras were purchased @ 100 GBP per unit. Once the installation has completed, the customer realises that the have 100 units left. They don't want to take them into stock, as its an outdated product spec now and return them to the supplier. However, the supplier will only accept / credit the customer at a discounted price for @80 GBP per unit. How do create the price variance against the PO or record the loss? Currently