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I am sorry to hear about this situation. They are bound by the contract just like you are. If they miscalculated the amount of money they needed, it is their own fault. Just like you could not change the amount after the fact, neither can they.
They must now fix the deck for that amount. If they do not, you can sue them for breach of contract in small claims court. To prevail in an action for breach of contract, a plaintiff must prove that the defendant owed the plaintiff a contractual obligation and that the defendant breached that obligation. See Continental Masonry Co., Inc. v. Verdel Const. Co., Inc., 279 Md. 476, 480, 369 A.2d 566, 569 (1977).
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