If you disclosed the asset in your petition
and at the 341 hearing, I'm a little confused as to why the trustee had to hire an attorney to "pursue" you for the money at issue - - and I certainly see that this is your basic confusion as well!
Trustees are notorious for doing junk like this - - and unfortunately - - most of them are able to get away with it. That said, I really think you have a defense the claim and may be able to prove to the court that you should have to pay for the trustee's attorney or other fees BECAUSE you've never contested owing the money and actually have disclosed it from the beginning of your bankruptcy.
However, the problem is an economic one - - and the question you'll have to ask is whether it would be cheaper to hire an attorney to fight (with no guarantee of winning) or to just pay the money? The trustee knows your predicament and that's why he/she isn't backing down.
Thus, it may be best to contact a local attorney, go over the facts and then decide how much it would cost you to protest the claim of the trustee. If your attorney things you can give it a shot for less than the claim, it may be worth a shot. However, if the attorney wants a $2000 retainer to fight, then it's probably easier - - from a business standpoint - - to just pay the money and move on. I know it's hard to pay the money when you think you're being wronged, but you have to look at it from a business standpoint - - not as a matter of principle.