Betting is appealing, both in Real Life and in fiction. It adds a sense of risk to otherwise normal proceedings, makes everything more interesting. Unfortunately, "gambling is fun" is a poor aesop, not to mention that it's hard to make money actually mean anything in a work of fiction. So there's an easy solution: Rather than the loser giving the winner money, the loser has to obey the winner's orders--either once or for an entire day. Examples of Wager Slave include: