Oh, my, really? Methinks we need a utility for this instead of rampaging about in DFDM.  No one thought of this before?

Oh boy.  That is "OK" for a "one off" but not a valid Procedure to use DFDM.  We have clients that have tight Audit Controls on their companies.  The answer is to do the Credit Memo on Account (Don't apply to the original invoice) and then apply the CM to the AR Invoice manually in Cash Receipt.  And if the customer has already paid, you need to address how to refund the money to them. 

I deleted my original response which suggested use of Data File Display and Maintenance based on the responses above.  

No, you are not able to apply a credit memo or adjustment directly to an eInvoice due to the link to Paya.  

Looks like some real-world circumstances were not accommodated in the design.  That's going to need a proper program fix (including how to handle the Paya side of things, to not run into problems with a credit and online payment colliding...).