The story of Bruce Willis's cameo on the show is actually quite different:
* He was a friend of the show's creators: David Crane and Marta Kauffman had worked with Bruce Willis on "The Whole Nine Yards" in 2000.
* He wanted to help the show: Willis was a fan of the show and wanted to support its success, so he agreed to make a guest appearance in a few episodes.
* He negotiated a unique fee: Willis was paid a reported $1 per episode, because he jokingly threatened to "sue" them if he wasn't the highest-paid guest star.
While it's fun to imagine a bet leading to his cameo, the reality was a mix of friendship, support, and a bit of good-natured humor.