Make a bet that gives you very bad odds, only 20% chance you'll win!
Place a large bill into a walle and have a spectator lock it in with a padlock.
Show 5 keys and explain that only 1 of these keys will open the lock. They can win the money if they open the lock. Mix the keys up and have them freely selected by 4 different people.
Each person attempts to open the lock and all fail to win the bill!
Finally the performer opens the lock with the final key that was not selected!