law: Indian Contract Act, 1872 
  32. Enforcement of contracts contingent
  
  on an event happening
  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.
   Illustrations 
  (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.