How to enter a Credit memo

Vendor issued us a credit memo.  But in that they deducted an PO amount which we need to pay and sent us the net credit amount.  How to record this in the system.

Eg:  $100 for the Credit.  There is a $20 PO we need to pay.  So the vendor issued us a Credit memo for $80 ($100 -$20) instead of issuing the Credit memo and invoice separately.

Does oracle payables allow us to enter this in one single invoice.  We are in 12.1.3.