What message does my customer see after I reject their payment proof?

When you click Reject, a Reject payment proof box asks for a reason, for example "amount incorrect". Click Reject proof to confirm.

Rejecting cancels the order. In My Orders, the customer's Payment proof section then says their payment couldn't be verified, shows your reason, and tells them the order was cancelled and to place a new order.

They can't resubmit proof on that cancelled order, so if the mistake is small, contact the customer first. If they paid part of the amount, refund it from your bank or UPI app.