Reinfort executed 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? describe.