A rancher and a horse trainer agreed in writing that the horse trainer may pick any one of the black horses from the ranch for the contract price $5,000. The horse will be delivered to the horse trainer's place. Later the horse trainer refused to accept the horse when the horse was delivered to the horse trainer. The horse trainer's argument is that prior to the wring, the parties orally agreed the contract price was $4,800, and the horse trainer may pick any one of the black or white houses from the ranch. How should a court determine this case?