There is a general rule in land based casinos that a gambler shouldn't take credits left on a slot machine by someone else. Basically, from what I understand, if there is still credit showing on a slot machine. Don't cash out or add to your money you put into the machine. Right? The question is if you find a cash out voucher resting on the machine itself or on the seat, can you legally use it?
It makes me imagine the story of finding fallen money, what should I do? Yes, it seems like an easy question for adults, but it's a lesson in young children's classrooms. And as far as I know the way the instructors are is finding the owner if possible, or looking for someone who might be able to help you return it (maybe police,...) Going back to OP's problem, I think many of us have different thoughts and as long as it doesn't cause anything bad, everything will be fine. But IMO, you should still search and report to the manager about it, or reset the game and reload conveniently to avoid unfavorable situations.