How Revocation of offer and acceptance is done in contracts?

A proposal/offer may be revoked at any time before the communication of its acceptance is complete as against the proposer, but not afterward.

An acceptance may be revoked at any time before the communication of acceptance is complete as against the acceptor, but not afterward.

‘A’ makes proposal to ‘B’ to sell his house at a certain price. The letter is posted on 1st of the month. ‘B’ accepts the proposal by a letter sent by post on 4th. The letter reaches ‘A’ on the 6th . ‘A’ may revoke his offer at any time before ‘B’ posts his letter of acceptance. (i.e.., on 4th, but not afterwards). ‘B’ may revoke his acceptance at any time before the letter of acceptance reaches ‘A’. (i.e.., on 6th, but not afterwards).