AP of purchase order issue

I have a PO, the goods has been deliveried, and payment has been clear by customer, Now, vendor sent a invoice to us, but the unit price is different from the po unit price. I would like to know then how to do next step. I can't change the po price because the goods receipt and delivered and clear payment. I know that MIRO has a serveral type I can use, like credit memo, subsequent debit/credit, which type i can use in my situration. Thanks!