Yes, you can cancel a SEPA Direct Debit payment as long as it hasn’t been sent to the bank.
You can also refund a SEPA Direct Debit payment. However, this procedure has a few risks involved. First of all, be aware of the fact that the status of the payment will be put to paid after one working day even though the payment can still fail at this moment.
Also be aware of the customer’s ability to perform a chargeback. When you refund the money to the customer’s account, but he performs a chargeback at the same time, the customer will be credited twice. It is very important to clearly communicate to your customer that you are refunding the money.