Banks can always try to undo a transaction - it's just not guaranteed to work. AFAIK, credit/debit card reversals are always reversible because if the merchant

Banks can always try to undo a transaction - it's just not guaranteed to work. AFAIK, credit/debit card reversals are always reversible because if the merchant doesn't have the money, it becomes their bank's problem to get the money or eat the loss. This works because the merchant is easily identifiable, well known, and has a reputation to uphold (at least to their bank). Other methods of transfer don't come with such contractual protection, but can still have best-effort.