This sounds incredibly fishy. Get a good attorney now and do this purchase properly and legally. Otherwise, it sounds like you are being set up to be screwed, to use the technical legal term.
The information provided in the question posed is not complete enough to provide accurate legal advice. More details are needed to really guide the questioner. My response is not intended to be taken as legal advice and does not establish an attorney-client relationship.
Caution!! Very risky and probably not a valid agreement for a purchase of land. The property you want to purchase must be specifically described in a way that it can be identified, which means the description of the property which the property appraisers office of your county uses needs to be in the contract. If the contract you agree to is not a valid contract, the seller might indeed, be able to sell the property to someone else. Without a doubt, you need to contact an attorney licensed to practice law in your state and find out exactly what the legal requirements for a real estate contract are. Even you pay the attorney a few hundred bucks for some legal advice or to review the situation, it will be worth it to protect your legal rights.
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