law: Indian Contract Act, 1872 
  65. Obligation of person who has
  
  received advantage under void agreement, or contract that becomes void
  When an agreement is discovered to be void, or when a contract
  
  becomes void, any person who has received any advantage under such agreement or
  
  contract is bound to restore, it, or to make compensation for it, to the person
  
  from whom he received it.
   Illustrations 
  (a) A pays B 1,000 rupees, in consideration of B's promising to
  
  marry C, A's daughter. C is dead at the time of promise. The agreement is void,
  
  but B must repay A the 1,000 rupees.
  (b) A contracts with B to deliver to him 250 maunds of rice
  
  before the first of May. A delivers 130 maunds only before that day, and none
  
  after. B retains the 130 maunds after the first of May. He is bound to pay A
  
  for them.
  (c) A, a singer, contracts with B, the manager of a theatre, to
  
  sing at his theatre for two nights in every week during the next two months,
  
  and B engages to pay her hundred rupees for each night's performance. On the
  
  sixth night, A willfully absents herself from the theatre, and B, in
  
  consequence, rescinds the contract. B must pay A for the five nights on which
  
  she had sung.
  (d) A contracts to sing for B at a concert for 1,000 rupees,
  
  which are paid in advance. A is too ill to sing. A is not bound to make
  
  compensation to B for the loss of profit which B would have made if A had been
  
  able to sing, but must refund to B the 1,000 rupees paid in advance.