Amy has been working for Bill for 15 years. Despite the fact that Bill runs a very profitable company, and that Amy is a very productive employee, Bill has not given Amy a raise in 15 years. Bill has long promised he will give Amy a raise sometime in the future, but never does. Amy has threatened to quit, but Bill ignores this because he knows quitting and finding a new job will pose significant inconvenience to Amy.
So Amy goes to a third party, a signs a contract with the third party. The contract states that unless Amy has received a raise of $2.00/hr by January 1, the third party is entitled to all subsequent wages she earns from Bill.
The third party then gives the contract to Bill. Bill sees that if this contract is enforceable, then Amy must be very serious. Bill is a rational businessman, and calculates that the cost of giving Amy her well-deserved raise is much less than bringing on a new, untrained employee.
The only question is: Is this enforceable?
EDIT: Amy pays the third party a small fee. The third party is a business that specializes in these types of contracts.
(Amy and Bill are not real people, so they are having trouble contacting an attorney in their jurisdiction.)