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Locate the payment transaction in the account activity, click it, look for a cancel button. The bank payment still has to clear before it is returned to the sender which may take several days. If there is no cancel button available yet, then you'll have to wait for the payment to clear before issuing a refund if the option to refund isn't also available yet.
They can reach out to their bank to see if a stop payment is available, but normally letting the funds clear and then refunding them or withdrawing them back to the bank is a faster process.
- Jon K