To credit an Invoice, use the Credit option on the Spill menu of a posted customer invoice.
This will create an identical copy of the original invoice with the quantities reversed.
Enter the Date for the Credit Invoice and Confirm.
You can now edit the credit note before saving, and then posting.
The original invoice and invoice date are noted at the top of the new invoice.
Once an invoice has been credited, the credit needs to be applied to an invoice. See Applying a Credit to an Invoice for the steps on how to do this.