Cancel Apple Pay

Found this on the Apple Pay for Merchants FAQ:

How do I process returns with Apple Pay?

Use the Device Account Number to find the purchase and process the return, just like you would with a traditional credit, debit, or prepaid card payment. In Japan, you can also use the transaction ID on the receipt to find the purchase and process the return.

To see the last four digits of the Device Account Number, ask the customer to go to Wallet, tap the card, and tap  on the lower-right corner of the display.

You can also have the customer hold their iPhone or Apple Watch near the reader, select the card that they used to make the original payment, and authorize the return with Face ID, Touch ID, or a passcode.

I would speak to the merchant about it.

Have you tried to cancel the order on the purchaser's website?

Are you referring to Apple Pay Cash?

If so, once the payment has been accepted, there is no way to reverse it. it's just like using cash.