law: Indian Contract Act, 1872 
  24. Agreements void, if considerations
  
  and objects unlawful in part
  If any part of a single consideration for one or more objects, or
  
  any one or any part of any one of several considerations for a single object,
  
  is unlawful, the agreement is void.
   Illustrations 
  A promises to superintend, on behalf of B, a legal manufacture
  
  of indigo, and an illegal traffic in another articles B promises to pay to A
  
  salary of 10,000 rupees a year. The agreement is void, the object of A's
  
  promise, and the consideration for B's promise, being in part unlawful.