I think you shouldn’t keep the $100 in check or cash.Lets say you find the money and someone is with you  and they convince you to keep it.Do the opposite.Be decent if they convenced you 100 time you still shouldn’t do it.You should call the store or some thing and tell them to call you and if someone comes and tells you how much it is.But if not you can keep it but atleast wate for 24 hours.So if so dont lie about it.