law: Indian Contract Act, 1872 
  51. Promisor not bound to perform, unless
  
  reciprocal promisee ready and willing to perform
  When a contract consists of reciprocal promises to be
  
  simultaneously performed, no promisor need perform his promise unless the
  
  promisee is ready and willing to perform his reciprocal promise.
   Illustrations 
  (a) A and B contract that A shall deliver goods to B to be paid
  
  for by B on delivery. A need not deliver the goods, unless B is ready and
  
  willing to pay for the goods on delivery.
  B need not pay for the goods, unless A is ready and willing to
  
  deliver them on payment.
  (b) A and B contract that A shall deliver goods to B at a price
  
  to be paid by installments, the first installment to be paid on delivery.
  A need not deliver, unless B is ready and willing to pay the
  
  first installment on delivery.
  B need not pay the first installment, unless A is ready and
  
  willing to deliver the goods on payment of the first installment.