law: Indian Contract Act, 1872 
  23. What consideration and objects are
  
  lawful, and what not
  The consideration or object of an agreement is lawful, unless -
  It is forbidden by law; or
  is of such nature that, if permitted it would defeat the
  
  provision of any law or is fraudulent; or
  involves or implies, injury to the person or property of
  
  another; or
  the Court regards it as immoral, or opposed to public policy.
  In each of these cases, the consideration or object of an
  
  agreement is said to be unlawful. Every agreement of which the object or
  
  consideration is unlawful is void.
   Illustrations 
  (a) A agrees to sell his house to B for 10,000 rupees. Here, B's
  
  promise to pay the sum of 10,000 rupees is the consideration for A's promise to
  
  sell the house and A's promises to sell the house is the consideration for B's
  
  promise to pay the 10,000 rupees. These are lawful considerations.
  (b) A promises to pay 10,000 rupees at the end of six months, if
  
  C, who owes that sum to B, fails to pay it, B promises to grant time to C
  
  accordingly. Here, the promises of each party is the consideration for the
  
  promises of the other party, and they are lawful considerations.
  (c) A promises, for a certain sum paid to him by B, to make
  
  goods to B the value of his ship of it is wrecked on a certain voyage. Here,
  
  A's promises is the consideration for B's payment and B's payment is the
  
  consideration for A's promise, and these are lawful considerations.
  (d) A promises to maintain B's child, and B promises to pay A
  
  1,000 rupees yearly for the purpose. Here, the promise of each party is the
  
  consideration for the promise of the other party. They are lawful
  
  considerations.
  (e) A, B and C enter into an agreement for the division among
  
  them of gains acquired or to be acquired, by them by fraud. The agreement is
  
  void, as its object is unlawful.
  (f) A promises to obtain for B an employment in the public
  
  service and B promises to pay 1,000 rupees to A. The agreement is void, as the
  
  consideration for it is unlawful.
  (g) A, being agent for a landed proprietor, agrees for money,
  
  without the knowledge of his principal, to obtain for B a lease of land
  
  belonging to his principal. The agreement between A and B is void, as it
  
  implies a fraud by concealment, by A, on his principal.
  (h) A promises B to drop a prosecution which he has instituted
  
  against B for robbery, and B promises to restore the value of the things taken.
  
  The agreement is void, as its object is unlawful.
  (i) A's estate is sold for arrears of revenue under the
  
  provisions of an Act of the Legislature, by which the defaulter is prohibited
  
  from purchasing the estate. B, upon an understanding with A, becomes the
  
  purchaser, and agrees to convey the estate to A upon receiving from him the
  
  price which B has paid. The agreement is void, as it renders the transaction,
  
  in effect, a purchase by the defaulter, and would so defeat the object of the
  
  law.
  (j) A, who is B's mukhtar, promises to exercise his influence,
  
  as such, with B in favor of C, and C promises to pay 1,000 rupees to A. The
  
  agreement is void, because it is immoral.
  (k) A agrees to let her daughter to hire to B for concubinage.
  
  The agreement is void, because it is immoral, though the letting may not be
  
  punishable under the Indian Penal Code (45 of 1860).