law: Indian Contract Act, 1872 
  70. Obligation of person enjoying
  
  benefit of non-gratuitous act
  Where a person lawfully does anything for another person, or
  
  delivers anything to him, not intending to do so gratuitously, and such another
  
  person enjoys the benefit thereof, the letter is bound to make compensation to
  
  the former in respect of, or to restore, the thing so done or delivered.
   Illustrations 
  (a) A, a tradesman, leaves goods at B's house by mistake. B
  
  treats the goods as his own. He is bound to pay A for them.
  (b) A saves B's property from fire. A is not entitled to
  
  compensation from B, if the circumstances show that he intended to act
  
  gratuitously.