Kelly runs a restaurant on State Street and enters into a deal with Ray for him to deliver fresh eggs and milk once a week. The parties agree on a price of $50 per delivery, plus the cost of food and sign a written agreement with each other that day. Recently, the cost of eggs has increased 10%, so Ray approached Kelly to request a 10% increase under the contract. Kelly agreed to this change. The next week, Ray delivered eggs and milk to Kelly's restaurant, but Kelly refused to pay the increase. If Ray sues, what would be his best argument?