If you want to cancel an invoice that has been paid with a SEPA run, follow these steps:
Go to the menu Financials > SEPA and click the menu 'More > Create return debit'.
Enter the booking number of the invoice that you wish to cancel and click on 'check'. You can now see the amount paid with SEPA direct debit.
Click on 'Continue'.
Now you can determine the details of the return debit (e.g. if the account was not covered, you can ask for a fee). It is important to select 'Open' as the payment method, so that the bill can afterwards be canceled. You also have to select a reason.
Confirm the settings by clicking on 'Create return debit'.
Now you can cancel the invoice as usual.
IMPORTANT - If you have collected the amount, you have to return it manually.