law: Indian Contract Act, 1872 
  5. Revocation of proposals and
  
  acceptances 
  A proposal may be revoked at any time before the communication
  
  of its acceptance is complete as against the proposer, but not afterwards.
  An acceptance may be revoked at any time before the
  
  communication of the acceptance is complete as against the acceptor, but not
  
  afterwards.
   Illustration 
  A proposes, by a letter sent by port, to sell his house to B.
  B accepts the proposal by a letter sent by post.
  A may revoke his proposal at any time before or at the moment
  
  when B posts his letter of acceptance, but not afterwards.
  B may revoke his acceptance to any time before or at the moment
  
  when B posts his letter of acceptance, but not afterwards.