You can initiate cancellation request via REST/SOAP from external system.

After receiving the cancellation request, input data is validated and also checked for duplicate. If the input request is not valid or it is a duplicate request, the cancellation request is rejected.

Once the cancellation request is accepted, a response message is sent out and the cancellation request is logged for further processing.

Once the cancellation request is processed fully - transaction is cancelled successfully or cancellation processing status is exception (couldn't be cancelled), the existing payment cancellation notification (XML) message is generated.