How can I refund money on a spent RetailVista gift card?

This QA is about refunding money on a RetailVista gift card. It is not possible to refund money on other gift card types (such as TCS, Intersolve, etc).

Within the RetailVista gift card definition, it can be set that 'Refund' is allowed, see the partial screenshot below.

If this is activated, it is possible to sell the gift card 'Negatively'. At that moment, the remaining balance of the gift card is read out and added as a sales line to the transaction. In the checkout screen, any payment method can be chosen to refund money on the gift card. Of course, items can also be purchased at the same time, which may result in a positive amount to be paid.