Cancel Apple Pay Transaction

The cancel option is fairly well hidden. You didn't say exactly where you looked so the following may be what you already tried, but may help if not. From Send, receive, and request money with Apple Pay - Apple Support

If you send money to someone who hasn't accepted the payment, you can cancel the payment. Here's how:

1. 1. Open Messages and go to the payment. Or go to the payment in transaction history. 
   2. Tap the payment to see its details, then check the Status field. 
   3. Tap Cancel Payment. The Status field updates to say Canceled. If you
   4. don't see the option to cancel, then the person has already accepted
   5. the payment. Try asking the person to send money back to you instead. 

If you reject or cancel a payment, the money will be refunded by the next business day depending on your bank or card issuer.