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Wow. I have heard some things before, but a calculated move to foil your sale? How childish!
To sue in a state court, one needs to have a "cause of action." There are numerous causes of action, such as "breach of contract," "negligence," "fraud," "unjust enrichment," etc., as well as causes of action rooted in statutory law. Every state has their own although they are very similar to each other in every state because they all stem from the same common law. A pleading in Court needs at least one cause of action, although it is not unusual to have more than one.
Here, the neighbor is doing several things which can be the basis of cause of action.
1) Civil trespass - self explanatory.
2) Defamation - the elements of a defamation claim are (1) The defendant published a statement; (2) The statement was defamatory concerning the plaintiff; (3) The defendant acted with (i) actual malice, if the plaintiff was a public official or public figure, or (ii) negligence, if the plaintiff was a private individual, regarding the truth of the statement WFAA-TV, Inc. v. McLemore, 978 S.W.2d 568, 571 (Tex.1998). By sending out false/misleading letters to third parties, then this applies.
3) Interference with Existing Contract - the elements of claim for tortious interference with prospective contract are: (1) There must be a “reasonable probability” that the plaintiff would have entered into the prospective relationship or contract; (2) An “independently tortious or wrongful” act by the defendant that prevented the relationship from occurring; (3) The defendant did such act with a conscious desire to prevent the relationship from occurring, or knew that the interference was certain or substantially certain to occur as a result of the defendant's conduct; and (4) The plaintiff suffered actual harm or damage as a result of the defendant's interference. Johnson v. Baylor University, 188 S.W.3d 296, 304 (Tex.App.-Waco 2006, pet denied). By sending out false/misleading letters to third parties with the aim to disrupt the sale, then this applies.
He can be sued for all three above, and a restraining order may be requested as part of the suit (or as stand alone). A letter to cease and desist will normally do the trick without needing to actually file suit, but, if he does not get scared away by the letter, litigation may be required.
Let me know if you need a sample letter.
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