That said, you should have an appraisal contingency... which says if the property does not appraise at the offered price thenyou can cancel and get your deposit back... HOWEVER, all of thse contingencies, though typical, are often negotiated away... so without knowing your specific situation it is hard to comment.
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Best of luck, and negotiate strategically, not emotionally.
However, I believe you are really asking a legal question about a contract. There is little you can do to prevent something that a contract provides as a remedy after you have signed it other than perform on the contract.
You want to review your contract with your Realtor and an attorney to understand whether you have an appraised value contingency in the event the appraisal does not come in. You also need to understand what remedies the contract provides if you are unable to perform on the purchase because the appraisal does not come in. Many builders contracts give them the right to retain any deposits put down, but does not necessarily limit their remedies to just that.
Again, I would review the contract with your Realtor and a good attorney to understand your rights and what the contract provides.