Tommy owes Sita $1000 for accountancy work that Sita did last month. Tommy calls Sita and explains he has lost his job and cannot pay the full amount already owed. Tommy offers to pay Sita $600 instead of the $1000, and Sita must promise not to ask for the remaining $400. Sita is a bit annoyed but promises to accept $600 instead of the $1000 already owed. Tommy transfers the $600 as soon as Sita agrees. The next day, Sita tells Tommy he must still pay the remaining $400. Under the rules of consideration, can Tommy enforce Sita's promise to accept $600 instead of $1000 or must he pay the remaining $400 to Sita? In your answer, make sure to use relevant case law to achieve the full marks!