law: Indian Contract Act, 1872 
  57. Reciprocal promise to do things
  
  legal, and also other things illegal
  Where persons reciprocally promise, firstly to do certain things
  
  which are legal, and, secondly under specified circumstances, to do certain
  
  other things which are illegal, the first set of promise is a contract, but the
  
  second is a void agreement.
   Illustration 
  A and B agree that A shall sell B a house for 10,000 rupees, but
  
  that, if B uses it as
  a gambling house, he shall pay A 50,000 rupees for it.
  The first set of reciprocal promises, namely, to sell the house
  
  and to pay 10,000 rupees for it, is a contract.
  The second set is for an unlawful object, namely, the B may use
  
  the house as a gambling house, and is a void agreement.