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. Illustrations A proposes, by a letter sent by post, 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 at any time before or at the moment when the letter communicating it reaches A, but not afterwards.