131. Preexisting Duty. Tabor is a buyer of file cabinets manufactured by Martin. Martin's contract with Tabor calls for delivery of fifty file cabinets at $40 per cabinet in five equal installments. After delivery of two installments (twenty cabinets), Martin informs Tabor that because of inflation, Martin is losing money. Martin will promise to deliver the remaining thirty cabinets only if Tabor will pay $50 per cabinet. Tabor agrees in writing to do so. Discuss whether Martin can legally collect the additional $100 on delivery to Tabor of the next installment of ten cabinets. (See Agreements That Lack Consideration.)