1. If you have over-billed the customer for example you sent him an Invoice for $110/-, the customer rings you back and says it should be $100 or ask you for a discount.
2. Customer returns stock to you. Say you Invoiced a customer for 12 pieces of Product A at $10 per piece. For some reason, the customer returns 2 pieces back to you. Your sales & debtor reports indicate $120 to this customer, and your Inventory shows 12 pieces out. You should not amend the Invoice since you already sent out the Invoice to the customer. Easiest way to rectify this is to issue a Credit Note for 2 pieces of product A. Note that if in your Invoice, you charged it to “Revenue – Sales for example”, you should charge it to the same account in the Credit Note. Do not enter negative values in the Credit Note as it is done automatically by the software.