In short, and what Phillip has told a thousand people previously, if you regged the name with prior knowledge of the .com or if you attempted to sell your domain to the owner of someone who has built some kind of intellectual property on the name (including the .com in this instance) then its irrelevant that he has no filed TM, you are a cybersquatter and yes, he can sue you for infringement.
Dont worry, its a mistake thats been made a zillion times before you. Your best best bet (unless this domain which youve registered is Uber generic) would be to say "hey ive had second thoughts, sorry for your trouble, here's the domain for free"