If a competent and experienced attorney correctly drafted your Separation Agreement, you should have a provision in it which discusses the ownership rights to the apartment, and when and if it is sold, how the proceeds will be divided. By maintaining her name on the ownership of such apartment, she has a legal right to half of the proceeds upon the sale, unless otherwise agreed in the Separation Agreement. However, you do have a claim for all the mortgage and maintenance payments which you have made during the time in which you have been divorced unless she can show that you intended to provide her a gift of such amounts which, under the circumstances, seems quite unlikely.
This issue may be mediated, and I strongly suggest that you contact me at 212-370-1660 to discuss mediating this matter in a civilized and cost-effective alternative to having this matter litigated in court.
Leonard M. Weiner, Esq./Divorce Solutions