The other answers are wrong.
Yes, you were able to depreciate your Leasehold Improvements.
Yes, you can write of the rest of the amount in the year that you abandon them.
EDIT: I think this next paragraph is incorrect for this situation; See comment later.
However, if you enter a sale price of $0, the program will treat it as a capital loss. But in the case of abandoning a Leasehold Improvement like you did, it should be a an "ordinary" loss (which is better for you). I strongly suspect that TurboTax is not set up for that. If that is the case, a work-around might be to first go to the rental section and say you converted that "asset" to personal use (that will keep the correct depreciation for the year). Then enter the sale in the "Sale of Business Property" section, but when you enter the purchase/placed-in-service date, enter a fictitious date that is less than one year before it was sold (which will then treat it as an "ordinary" loss rather than a "capital" loss).
