Trust me, it isn't the "Casino's money" it is always the other players money. At its most simplest, people come into the Casino and drop money on the floor, the Casino collects it, and then randomly gives some of it to people as they exit. It preferentially (by giving a higher probability) gives money to people who have come in a lot and have dropped a lot of money on the floor. At no time is there any risk that the Casino won't keep all of its "fraction" of the dropped money.