Seller and Buyer entered into a written contract for the manufacture by Seller of 10,000 widgets of a design specified by Buyer and set out in the contract. The contract also provided: “This contract may only be modified in a writing signed by both parties.” Before Seller began work on the widgets, Buyer and Seller agreed by telephone to a change in the specifications for 2,500 of the widgets. Seller then produced and delivered the 2,500 widgets as specified. Buyer refused to accept them because they did not conform to the specifications in the original contract. Assuming the CISG applies, who breached?