To record a refund, you must first access the reservation detail, and then click on "PAYMENTS", where you have already registered the guest's payment.
☞ Docs: ZaK - Reservation detail
In the following example, we already registered a payment of 202€. Here you can also find refunds already registered or you can record a new one by clicking on "ADD A REFUND":
In the example, I'm recording a refund of €50, paid in cash on 23/11/2021:
The refund will be displayed in "PAYMENTS" section, below payments already registered:
NOTE: Once recorded the refund, the "Unpaid" will be automatically updated. So the refunded amount is added/deducted from the recorded payments and it changes the amount to be still collected.
In the example, I registered a 50€ refund, so the Unpaid became 50€ and the Unpaid from 202€ became 152€: