David owns a music store. He wants to encourage more customers to visit the store so he decides that he will put in massage chairs for customers to use when listening the samples of music. David enters a contract where he will hire 5 massage chairs from Emily for a price of $300/month for six months. Three months later, David discovers that the massage chairs are not bringing any additional business. David talked to Emily about this. Emily promises that she will reduce the price of the chairs to $200/month for the remaining three months of the contract. Six months later David receives the invoice from Emily for the full amount of $300/month x three months, not the $200 x three months he was expecting to pay based on the promise made by Emily. David is happy to pay the reduced amount, but does not think that he is required to pay the full amount. Advise David.