There are many different payment scheme out there, which all have different parameters. ACH is someone unique in that money can just be pulled back, and even i

There are many different payment scheme out there, which all have different parameters. ACH is someone unique in that money can just be pulled back, and even in transactions that take a long time to clear, doesn’t mean that you can cancel them. Clearing time is usually caused by multiple sequential systems at multiple institutions taking their time do something. But once Citi started the transaction, and their system send the payment messages they probably couldn’t retract the payment. Once the first payment message was sent it immediately created a liability on Citi for the money (either to the payment scheme or the receiving bank), at that point actually moving the money becomes a bit academic, an unbreakable promise has already been made.