OK, good.
To paraphrase Yoda, there is no "going to sue" there is only sue or not sue. It's cheap and easy to get a lawyer to write a letter. Not so cheap or easy to file a lawsuit. Until you get served a summons and complaint you are free to ignore the threats. Or, you are free to succumb to the threats. Up to you.
Of course they do. You would be foolish to pay them anything at this point.
An idle threat per that statute I quoted.
If you do end up getting sued, one defense is lack of mitigation. The Texas Court of Appeals explains mitigation:
Section 91.006 of the Texas Property Code requires a landlord to mitigate his damages after the breach of a lease and declares void any lease provision to the contrary. See TEX. PROP.CODE ANN. § 91.006 (Vernon Supp.2005); Lunsford Consulting Group, Inc. v. Crescent Real Estate Funding VIII, L.P., 77 S.W.3d 473, 476 (Tex.App.-Houston [1st Dist.] 2002, no pet.); see also Austin Hill Country Realty, Inc. v. Palisades Plaza, Inc., 948 S.W.2d 293, 299-300 (Tex.1997), abrogated, in part, by TEX. PROP.CODE ANN. § 91.006.
The tenant bears the burden of proof to demonstrate that the landlord has mitigated or failed to mitigate damages and the amount by which the landlord reduced or could have reduced his damages. See Austin Hill Country Realty, 948 S.W.2d at 299.
A tenant's contention that the landlord failed to mitigate his damages must be pleaded as an affirmative defense. See Austin Hill Country Realty, 948 S.W.2d at 300. However, when a tenant contends that the landlord has actually mitigated his damages, the breaching tenant 278*278 need not plead the landlord's actual mitigation as an affirmative defense. See Austin Hill Country Realty, 948 S.W.2d at 300. Rather, the tenant's evidence of the landlord's mitigation tends to rebut the measure of damages under the landlord's claim of breach and may be admitted under a general denial. See Austin Hill Country Realty, 948 S.W.2d at 300. The offset of a landlord's breach of lease damages by the amount he was able to mitigate his damages raises a fact issue regarding the reasonableness of the offset, precluding summary judgment. See Nautilus Training Ctr. No. 2, Inc. v. Seafirst Leasing Corp., 647 S.W.2d 344, 347 (Tex. App.-Corpus Christi 1982, no writ).
Google Scholar
There are many other Texas case decisions involving that statute:
91.006 - Google Scholar
It would be a good idea for you to consult a real estate attorney regarding mitigation and see if it's worth responding to the landlord's attorney with an admonition to mitigate or to wait until a lawsuit occurs and then sandbag the plaintiff with a mitigation defense.
Meantime, I suggest you take dated photos of the "For Lease" sign and check Craigslist and your local paper's real estate ads to see if the landlord is doing any advertising.
Another option is to negotiate a lump sum full and final cash settlement (in writing, of course). Best to have a lawyer do that for you.
Your leverage is holding on to your money while your enemy isn't getting any.
Also understand that the above is not legal advice and if you act on my comments you do so at your own risk.