First of all, you can file amended tax returns for past years. Use an accountant. It will trigger an audit but if you provided her support and he has provided nothing, then you may be entitled to the exemption. If on the other hand, you have been unemployed, does it make a difference that he claimed her? Review his financial affidavit and supporting documents. Try to ensure your daughter gets health insurance through his employer. Consult an attorney so you know whether you are entitled to alimony, unequal equitable distribution, and similar items. Do not sign anything unless you are certain it settles everything as it is binding.
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