@W012 

Yeah, I agree that your solution is clean and simple.  You're not missing anything. 

The other option is to create a quick journal entry (JE): debit A/R for the customer and credit Sales Returns & Allowances.  That prevents you from having to set up a new A/R Payment Adjustment service item.  I try to avoid JEs whenever possible, but having to set up a service item for most likely one-time use is an exception.

I'm sure you know this, but don't forget to apply the outstanding credit to the A/R balance created by the invoice/JE to close it.