Reinfort executed a written contract with Bylinski to purchase an assorted collection of shoes for $3,000. A week before the agreed shipment date, Bylinski called Reinfort and said, ‘‘We cannot deliver at $3,000; unless you agree to pay $4,000, we will cancel the order.’’ After considerable discussion, Reinfort agreed to pay $4,000 if Bylinski would ship as agreed in the contract. After the shoes had been delivered and accepted by Reinfort, Reinfort refused to pay $4,000 and insisted on paying only $3,000. Is the contractual modification binding? Explain.