Agreement cases in the law of contract refer to legal cases that involve disputes over the existence or validity of a contract between two or more parties. These cases typically hinge on whether the parties reached a mutual agreement, which is a necessary element of a binding contract.

One of the most famous agreement cases is Carlill v. Carbolic Smoke Ball Co. In this case, the Carbolic Smoke Ball Company advertised a product that they claimed could prevent users from catching influenza. The advertisement included a promise to pay £100 to anyone who used the smoke ball as directed and still caught the flu. Mrs. Carlill purchased the smoke ball and followed the instructions, but still caught the flu. When she sought the promised payment, the company refused, arguing that the advertisement was not a binding offer. The court disagreed, finding that the advertisement constituted an offer to the world at large, and Mrs. Carlill had accepted that offer by purchasing and using the smoke ball as directed.

Another important agreement case is Smith v. Hughes. In this case, the plaintiff Smith was a prostitute who entered into a contract with the defendant, Hughes, to provide her services as a prostitute. However, Hughes argued that he had not intended to enter into a contract with Smith, but had merely wanted her to stand in a window so that he could look at her. The court found that this intention was irrelevant – what mattered was the external objective manifestation of agreement. In this case, the parties had reached a mutual agreement, and therefore a contract existed.

A more recent agreement case is the case of Blue v. Ashley. In this case, the defendant Ashley, a football club owner, had made a promise to the claimant Blue that he would pay him a £1m bonus if he successfully helped the club avoid relegation. However, when the club did avoid relegation, Ashley refused to pay the bonus, arguing that the promise had been made in the heat of the moment and was not a binding contract. The court disagreed, finding that the parties had reached a mutual agreement, and Ashley was therefore liable to pay the bonus.

These cases serve as important reminders of the importance of agreement in the law of contract. In order for a contract to be binding and enforceable, the parties must have reached a mutual understanding and intention to be bound. Whether it is an advertisement, a promise made in the heat of the moment, or a more formal arrangement, the key is that both parties must agree to the terms of the contract. As a professional, it is essential to pay close attention to the details of these cases and ensure accurate and clear communication of legal concepts.