If you would like to cancel an invoice you click on the three dots in the overview and select 'Cancel invoice'.
Here it is important that you state a reason why this invoice is cancelled - e.g. 'wrong invoicing'. With 'Cancel invoice' you will confirm the step. In the tab 'Payment status' the notification 'cancelled' will appear in the status 'paid' and the amount will be removed from the turnover.
If the invoice was paid via online payment (i.e. Creditcard, PayPal) the amount will be refunded automatically.
If the invoice was paid with auto-SEPA you cannot cancel it anymore.
The customer must first cancel the transaction, after that you can delete the invoice.