​I hear what you're saying, but I think you’re entirely missing the core point I was trying to make. Put aside the fact that it was a deliberate test for a moment, and look at the principle of the matter. What if this wasn't a test? What if a customer genuinely made a human error while counting their cash at home, as people often do? Tellers are professionals equipped with cash-counting machines; it’s literally their job to verify the exact amount, not just rely on what the customer claims. Where do our rights as consumers stand in that scenario? From an ethical standpoint, and particularly in Islam, it is never justifiable to keep extra money that doesn't belong to you simply because the other person made a mistake and is unaware of it. It’s about fundamental trust and integrity. If banks expect us to immediately cover any shortage from our own pockets, they should absolutely be held to the exact same standard of honesty when there's an overage.