If the attorney failed to file the papers despite knowing that he had to and telling you that he would, you may have a malpractice case against the attorney for the increased amount of taxes you had to pay ($2,600). Since bringing a malpractice case can itself be costly, however, you are best off trying to settle this with the attorney without litigation (i.e. contacting him, explaining what happened, and stating that you believe that under the circumstances, the lawyer should cover the increased bill). This may mean accepting only part of what you feel you are owed, but that is most likely better than having to go through litigation. If you can't settle it, you can sue.