To make a refund, in the portal go to Transactions.
Look for the original transaction there.
Select the transaction and click on the Refund button.
You can then enter the amount that you will refund to the customer.
This amount can never be higher than the original transaction.
You will recognise a refunded transaction in your Kassa Compleet portal transaction overview.
Because the R letter is displayed. You can view the refund by clicking on it.