On June 1, Buyem, Inc., a widget manufacturer, entered into a written agreement with Mako, Inc., a tool maker, in which Mako agreed to produce and sell to Buyem 12 sets of newly designed dies to be delivered August 1 for the price of $50,000, payable ten days after delivery. Encountering unexpected expenses in the purchase of special alloy steel required for the dies, Mako advised Buyem that production costs would exceed the contract price; and on July 1 Buyem and Mako signed a modification to the June 1 agreement increasing the contract price to $60,000. After timely receipt of 12 sets of dies conforming to the contract specifications, Buyem paid Mako $50,000 but refused to pay more. Which of the following concepts of the Uniform Commercial Code, dealing expressly with the sale of goods, best supports an action by Mako to recover $10,000 for breach of Buyem's July 1 promise? 
Bargained-for exchange. Promissory estoppel. Modification of contracts without consideration. Unconscionability in the formation of contracts.