"Mistakes cannot be rectified after you leave the counter"
That is all.
This. If it happened the other way round, you couldn't have gone to then saying they had made a mistake and expect to get your money back. So it shouldn't be expected for the reverse to happen either.
Also, what sort of a job will sack someone over making a mistake like that? While it does partly depend on the amount of money lost, even so I strongly doubt something like that is even a sackable offence.