The custodial parent could ask the court to modify the support order to deprive you of the right to take the exemption. However, if the custodial parent has already executed an IRS Form 8332 for the 2011 tax year, then it's too late, because the IRS will not accept a revocation until the tax year after the year in which the revocation is signed.
The court could modify your support obligation to pay the difference in taxes to the custodial parent as a means of offsetting your tax benefit. Regardless, the only way to accomplish this is for the custodial parent to file a motion to modify support with the court. Otherwise, if the current orders state that you are entitled to the tax exemption, then that's the "law of the case."
Sometimes people think a law exists, when it does not. You can always ask the other parent to "please cite the legal authority supporting the revocation of your right to the exemption." Most of the time, the response to this sort of request is the sound of crickets chirping. But, again, the parent can ask the court to modify the support obligation, so things could still get ugly, and end up not in your favor.
Hope this helps.
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