There are two cases that we need to issue a credit invoice to customer. One to cancel a previously been issued invoice and the other just to create a credit note. A credit should be the same as the invoice including quantity, unit cost, tax option. The only difference is that it counts negative on customer balance.
Lets say we have charged the customer for 1 Year Domain name and 1 Year web hosting. We have already sent the invoice to customer and already sent the invoice to the TAX Authority. For legal purposes we cannot cancel that invoice unless we issue a credit with the same data and also the VAT. A payment or an in-house credit is not correct because we need to claim the VAT back.
The ideal would be to press a button and marked the invoice as revered and automatically issue the credit invoice with the same data as the invoice. This would be one option. The other option is to create a completely new invoice. Normally you need to credit part of or all the invoice.
See attached a sample screenshot for the Credit