If you need to provide a refund for a customer for an item on an Invoice, but not return the item to stock, you can Copy To Credit Memo and select a Whse marked as Drop Ship to reverse only the accounting side of Revenue and Customer amounts.
For example: If I add an A/R Invoice for 10 items at $5, then I need to refund the customer for an item (but not return it to stock).
I can go to the Invoice, Copy To A/R Credit Memo
Then, select a warehouse that is marked as Drop Ship. Add the Credit Memo
This will only reverse the Revenue posting and the customer posting, but not return the item to stock or effect inventory values.