When you work with the SEPA downloads, you can process the results in our system.
Normally, when a client reverses a payment but you still try withdrawing it, you can process this reversal with a certain error code in the SEPA results. The invoice will automatically be changed back to outstanding so you can try again in the next batch.
Since your clients have time to reverse a payment that you've tried withdrawing from your account, their invoice may already be set to "paid" when the reversal comes through.
To process a reversal after the invoice is "paid", go into the invoice and delete the payment there. Click the trash can next to the direct debit payment and the invoice will be set back to outstanding. However, you will not be able to see that the invoice was reversed and it will just look like an ordinary outstanding invoice.