Buyer and Seller entered into a written contract for the manufacturer by the buyer of 10,000 digits 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 and signed by both parties". Before the Seller begin work on the widgets, the Buyer, and Seller agreed by phone to change in the specifications for 2500 of the widgets. The seller then produced and deliver the 2500 widgets as specified. The buyer refused to accept them because they did not conform to the specifications in the original contract. Assuming the CISG applies, who breached the contract?