Credit Memo base on amount

Hi need to applied a credit memo to an invoice that is already matched agains PO (Receipt Option), we are using the Matching option Receipt, but we would like to make adjustment to the invoice amount, not to an specific line, prince o quantity billed, but if we use the option "Amount Correction" no invoice or po appear or is available, only quantity and prince show information, but if we use this we had a incorrect price variance

If it possible to use the Amount Correction option to reduce only the Liability??