An offer can be withdrawn at any time before it is accepted, unless there a contract to keep it open; which must be supported by consideration.
Withdrawal must be communicated to the offeror. The postal rule does not apply to offers (or withdrawals), only to acceptance. In Byrne v Van Tienhoven , a postal revocation was sent before an instantaneous telegram of acceptance. As the acceptance was received before the letter of revocation, there was a contract and the postal rule did not apply. In Dickenson v Dodds , it was ruled that if the offeree knows of revocation, that is sufficient; an explicit communication does not need to occur. We must also distinguish between counter offers and mere inquiries, as in the case of Stevenson v McLean .
To revoke a unilateral offer, the offeror must take reasonable steps. Both a significant lapse of time or death will also revoke such offers. In Errington v Errington , it was ruled that once performance had started, there was a collateral contract keeping the contract open to its beneficiaries. Later, in Dahlia v Four Millbank Nominees , it was also ruled that unilateral offers cannot be revoked once performance has started.
Finality of agreement
A contract may not be completed with uncertain terms. May and Butcher v R  ruled that the term ‘a price to be agreed’ could not complete a contract, however a court could have imposed a reasonable price if nothing had been said about the price...
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