If you click through to the payment in question you should see a “refund balance” option at the top, which lets you refund any amount up to the total unallocated amount on the payment. Given you’re going to immediately put the money back onto another client you don’t really need to create a separate bank account (unless this is something you’re likely to do on a regular basis), you could just refund to your drawings or director loan account, then create a payment for the other client on the same date from the same account.