Contingent contracts to do or nor to do anything in an uncertain future event happens, cannot be enforced by law unless and until that event has happened.
If the event becomes impossible, such contracts become void.
(a) A makes a contract with B to buy B’s horse if A survives C. This contract cannot be enforced by the law unless and until B dies in A’s lifetime.
(b) A makes a contract with B to sell a horse to B at a specified price, if C, to whom the horse has been offered, refuse to buy him. The contract cannot be enforced by law unless and until C refuses to buy the horse.
(c) A contracts to pay B a sum of money when B marries C. C dies without being married to B. The contract becomes void.