This is a user to use forum, not Apple support. However, do look at Send and receive money with Apple Cash - Apple Support which states:
How to cancel or dispute a payment
- Open the Messages app, then open the conversation and tap the payment.
- Your Apple Cash card will open in the Wallet app. Under Latest Transactions, tap the payment, then tap it again.
- Tap Cancel Payment.
If you don't see Cancel Payment, the person has already accepted the payment. Try asking the recipient to send the money back to you instead.
It can take 1-3 business days for the money to be refunded, depending on your bank or card issuer.
Apple Pay person to person payments are just like any other private transaction between two people, so only send and receive money with people you know. If you have questions or concerns about a transaction, please contact us.
Avoid scams when you use Apple Cash - Apple Support