Head over to "Payments > Transactions" and click on the transaction you want to refund. At the top left, you will see the "Refund" button. Click it and decide which amount you want to refund. At first, the same value as the purchase is already typed in.
By customizing the refund amount, you can repay parts of the price. Refunding more money at later stages when a partial refund has already taken place is always possible.
Refunds directly from Payrexx are only possible with selected payment providers.