What do you do with an Aldi's cart that was given to you without paying?

If you're given a cart at Aldi rather than using your own quarter, when you're done shopping do you hand off your cart to the next person or keep the quarter for yourself, while getting a quarter from the cart next to you? There was some weird grump at Aldi earlier who got all defensive about, getting his money back when it wasn't even his to begin with.