Edward cannot enforce Malcolm's promise that the car was 'sound and in good condition' because Edward's consideration (the payment for the car) was already past when Malcolm made the promise. In contract law, for a promise to be enforceable, there must be consideration, which means something of value must be exchanged at the time of the promise or in the future, not in the past. The case of Roscorla v Thomas (1842) 3 QB 234 is relevant here, establishing that a promise made after the consideration has already been given is considered past consideration and is not valid for enforcing the promise. Since the car sale was completed before Malcolm made his promise, Edward cannot enforce it as there was no new consideration given for this subsequent promise. 

Key Point: Consideration must be present or future and cannot be based on past actions. This principle ensures that enforceable promises are supported by a mutual and current exchange of value. Any promise made after the initial consideration is typically unenforceable unless new consideration is provided.