See the following instructions from https://support.apple.com/en-us/HT207875
If you send money to someone who hasn't accepted the payment, you can cancel the payment. Here's how:
- Open Messages and go to the payment. Or go to the payment in transaction history.
- Tap the payment to see its details, then check the Status field.
- Tap Cancel Payment. The Status field updates to say Canceled. If you
- don't see the option to cancel, then the person has already accepted
- 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.