A cancelation is performed using a previous payment authorization (AUTH), by referring its transactionID and sending a POST request over HTTPS to the /payments/{transactionID}/cancelation endpoint. 

When canceling a payment and, if sent within the time-frame, the authorization is not captured yet, instead it causes an authorization reversal request to be sent to the card issuer to clear the funds held against the authorization. If requested over a MULTIBANCO Reference generated but not paid, the reference will be cancelled. 

POST /api/v1/payments/{transactionID}/cancelation