Is the vendor credit for a price adjustment only or was material (inventory returned) for credit?
 
AP-C does allow for an invoice to be revised - if it is a price only adjustment, one suggestion would be to revise the original invoice amount for the credit issued (this assumes the original invoice has not been paid).
 
If inventory has been returned and the vendor has granted a credit. then the subject purchase order has to be revised to show a negative qty amount being returned to vendor.  This negative qty has to be received against the po (PO-C) and then the credit appears in AP-C to which the credit memo can be applied.

Not an easy one but yes.  Create a PO with 2 lines - a negative at the incorrect price and a positive at the correst price.  Receive both lines and the net result to AP-C will be a credit but you will have to enter it in 2 parts if the net is negative.  Enter an invoice for the positive and a credit for the negative.  That will pass the net credit amount to AP.