law: Indian Contract Act, 1872 
  52. Order of performance of reciprocal
  
  promises
  Where the order in which reciprocal promises are to be performed
  
  is expressly fixed by the contract, they shall be performed in that order, and
  
  where the orders is not expressly fixed by the contract, they shall be
  
  performed in that order which the nature of transaction requires.
   Illustrations 
  (a) A and B contract that A shall build a house for B at a fixed
  
  price. A's promise to build the house must be performed before B' s promise to
  
  pay for it.
  (b) A and B contract that A shall make over his stock-in-trade
  
  to B at a fixed price, and B promise to give security for the payment of the
  
  money A's promise need not be performed until the security is given, for the
  
  nature of the transaction requires that A should have security before he
  
  delivers up his stock.