If your spousal agreement states that he may claim your daughter this year as a dependent IF he is paid up on child support, then if he was not paid up as of December 31, 2008, he would be considered delinquent on payments for the tax
year of 2008. In that case, you should have the right to claim your daughter on your 2008 tax return. Also, the spousal agreement which you refer to must be signed by both you and your ex-husband in order for it to be valid and accepted by the IRS.
In the event that your ex-husband also tries to claim your daughter, the IRS will request documentation from each of you to determine who has the legal right to claim your daughter. Typically the IRS would grand that right to the custodial parent, or the parent who had custody of the child for more than half of the year. However, if there is a written legal agreement in place to determine who is granted this exemption, then that will take precendence.
In order to protect yourself in this situation, you should get a documented statement of account from the support division that shows the amount of child support that was in arrears as of December 31, 2008. In the event that both you and your ex-husband claim your daughter, you will need such a document to justify why you claimed the exemption.
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