Fully credit a single invoice
To generate a credit invoice for the full amount, click the button in the upper right corner when viewing the original invoice you want to credit.
Partially crediting an invoice
If you only want to credit only a part of an invoice, go to a customer detail page and scroll down to the the Invoices section.
In the Invoices section there is a "Create Invoice" button. For a partial credit, enter the amount to invoice with a minus sign (so -5.00 for 5 euro credit). This will generate a negative invoice in your bookkeeping.
If you are are looking for more information on how to issue refunds, please review the following article: Issuing refunds