How do I undo an Apple Pay transfer?

According to: Transfer money from Apple Pay Cash to your bank account - Apple Support

1. "Enter your bank information carefully. If you enter the wrong information, your money might go to the wrong account and may not be able to be recovered. If you need help, contact us." 

Be sure to click on the "contact us." link.

Tap the three tiny dots LR corner to see the transaction and options.

Rescind.