It is possible to change the payment method after the payment has already been made. In order to change the payment method, you need to navigate to the Journal and mark all tickets in question.
Then select Edit - Change payment or simply klick on the icon and choose the correct payment method in the dropdown menu.
Furthermore, you have the option to split the payment, by selecting the amount of the second payment method additionally to the original one.
Having set the second payment method, the entry in the journal will read Zahlungssplit, where all payment methods considering the booking can be viewed by Data - Total amount (Ctrl + G) or by clicking the icon .