13-1. 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 cabi- nets), Martin informs Tabor that because of inflation, Martin is losing money. Martin will promise to deliver the remain- ing 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.)