Not an attorney but will share what I know.
A party doesn't actually need a registered TM if they can prove common law,
although having a registered one gives leverage (to a certain degree).
Your strongest point, based on what you posted, is how your negotiation with
the other party occurred. They may try to argue what you did was bad faith,
but I doubt it'll hold water because
they initiated it and there was no
agreement concluded.
Your challenge, thus, is to prove you never used it in bad faith and/or have a
legitimate use for it. Just win one of the 3 required UDRP conditions, the other
has to win all 3.
However, even if they lose, they still have the option to sue you in court. But
so far that appears doubtful based on the additional details you mentioned.
Here are some sites you can read more on the subject:
http://chillingeffects.org/trademark/
http://chillingeffects.org/domain/
http://gigalaw.com/articles/domainnames-trademarks.html
Just the same, though, be prepared with legal counsel. If you need reliable
counsel, just shoot a blank email to attorneyATdavezanDOTcom, you'll get 2
of the best.
Hoping this helps. BTW, this is best posted in the legal forum section...