If a person accidentally underpaid his fare on a bus, and did not realize until later, what is the appropriate tikkun? Do we say that since the bus driver did not say anything to him, he is relying on the goy’s error – which is not considering stealing?
From your question I understand that you didn’t do anything to mislead the bus driver, then presumably it will be tous akum, and you don’t have to return it. However if returning it will make a kiddush hashem them it is a good thing to do, for example if you have this bus driver daily, then you could return it to him. Remember though to get a receipt for the money, otherwise it will end up going to the bus driver, and not the company.