ANSWER:

You must go back and give him the rest of the money.

It might be cheap but Jannah is not! You don’t want to be held accountable for it on the day of judgement when you know that he had made a mistake. 

I bought 5 lolipops and i later noticed that the cashier charged for only 4 lolipops, am i sinful if i don’t go back to the store and tell the cashier? Since it’s pretty awkward and lolipops are so cheap.