Harry could argue successfully that he revoked the offer effectively. Mary needs to be prove the 'Postal Rule'. It is said to be a rule of convenience. 'Acceptance is valid when posted even if the letter is incorrectly addressed or lost and never arrives' this rule is well illustrated in Adams v Linsell where the defendant wrote to the claimants offering to sell wood and wanted a reply 'in course of post'. On 5th September the offer reaches the claimants and then on the 8th the defendants sell the wood to a third party.
On the 9th September acceptance letter from the claimants arrive. It was held that there was a valid contract because acceptance was made on the 5th September when the letter was posted not when it arrived on the 9th. However there are exceptions to the postal rule. The first exception is; if a letter is handed to a postman it is not posted as he is only authorised to deliver post. The next exception is; if a letter is not properly addressed it is not valid.
The next exception which is relevant to this situation is in the case of Holwell Securities v Hughes where the legal principal was established that if an offeror requests to see the acceptance then it is not valid until it is received by the offeror. Following the decision in Adams v Linsell Mary would have a valid contract under the postal rule as she sent her acceptance on the 6th and acceptance is valid when sent not received. However relating the exception established in Holwell Securities v Hughes the exception does apply and the postal rule cannot be used.
Harry has excluded from the unilateral contract the postal rule with the wording 'first 12 replies received'. He has stipulated that he wants to receive the acceptance so it is not valid when it is posted it is valid when it is received. Harry did not receive Mary's acceptance until 7th January which is too late as the offer had been revoked on the 6th January. It is up the courts as to whether Harry can rely on the postal rule or not.
Mary's offer did not correspond exactly with the offer that was made. Harry's advert asked for a i?? 75 cheque, whereas Mary sent 75 cash. This could be a counter offer as acceptance should be an unqualified acceptance of all the terms of the offer. The leading case is Hyde v Wrench, where on the 6th June the defendants offered the claimants a farm for sale fro i?? 1000 and the claimants made a offer of 950.
On the 27th June the defendants rejected the counter offer, then on the 29th the claimants made a purported acceptance of the 6th June offer of 1000. it was held that there was no contract because the original offer to sell had been ended by a counter offer of 950.
As in this case and other counter offer cases the actual counter offer is normally when a lesser sum of money is offered instead. That isn't quite the same issue here. The difficulty is not that a lesser amount was offered but that the exact method of payment was not met. There are no leading cases currently where this kind of situation has occurred. It would be at the courts discretion whether Mary's acceptance was valid or void, due to the fact that it did not correspond exactly with the offer.