I would contact the casino and tell them. Sometimes i got more money in my paycheck from the company i worked for and ofcourse i would tell them. Its same her, this money doesnt belong to me and not only would it feel really bad just like steeling but also one day they might see this and want it back. I really think we should go back to kindergarden in our minds sometimes and remeber to put things back where we took them and not to take toys that belong to somebody else. You can also think about it like this: If the casino took "x amount" of money from you by misstake wouldnt you want them to give it back?