Am I allowed to stop/reverse/cancel a transaction after I have completed payment via CC on UPI?

No, after you initiate a transaction via a UPI App, you cannot cancel the transaction. After initiating the transaction, the UPI will prompt you to check the payment details.

Note – In case goods or services against the payment has not been received, in such scenarios cardmembers may dispute the transaction by calling Bank’s Customer Services at 022 6232 7777.