Cancelling a payment made with your phone is the same as a physical card. You may need to go back to the merchant you made the purchase from and cancel the payment.
If you are asked for the last 4 digit of your card, the last 4 digit of your Token or Device Account Number should be used instead of the last 4 digit of your physical card. To locate your Device Account Number: Launch Apple Pay, click "i " at the bottom right of your card. Look for Device Account Number.