Credit Memo for Service PO

Here are the details of my example:
PO with one amount-type service line for $1000.
Invoice comes in for $1000 and is fully matched and paid.
PO Line is now "closed" but PO190 not yet ran, so header is still open.
We should have only been invoiced partially, let's say for $500.
Credit memo for $500 comes in.

So, what I have done is entered the credit memo information, including the information on the Misc. tab, selecting the original invoice. Then, go to MA42.1 - Memo Line Detail - Purchase Order. Here, I'm attempting to add a line for the Cost Adjustment of Quantity = 1 and Unit Cost = $500-. Here is where I'm stuck: I get an error "Cost credit not allowed if no cost difference exists."

How do I match this credit memo to my invoice and po-line? What am I missing?