How to revise PO after payment was paid?

Hi all,

Could you please advice solution as scenario below?

Scenario:

I created the PO with project/task and mathced the invoice and the payment was paid (100 USD).

1 month later, supplier give the discount amount without return material on PO (-15 USD).

What should I do in the system to support this case by not void the payment nor cancel the invoice.

My requirement is 1) PO Value deducted to 85  2) Project cost is reduced.

For more information, please find as document attached.

Thanks and regards.

Pathikorn