Assuming that you would give it back , let's say you leave the shop and subsequently realise you have been given extra change . Let's also say it's $50 (a large enough amount to inconvenience the shop keeper) and it's just a small local vendor .
What is the accepted distance from the shop that you must travel before you no longer feel obliged to return ?
On vending machines....I'm keeping it , you're keeping it . Period.