Hi A.,
Probably not, unless he has some super duper good reason that I don't know about. I agree with the other mama's to get it in writing. You may have included it, and depending on what kind of state you were in when you guys filed I know lots of mama's that either didn't pay attention to some of the fine print or didn't understand the implications of letting the non-custodial parent file...... so you may have given him the abililty to claim her without knowing it.
If it's NOT written in the custodial decree, then the federal government says this.......
(basically the parent that the kid lives with most gets to claim them as a dependent unless that parent agrees to let the other parent claim the child).
http://www.irs.gov/pub/irs-pdf/p4449.pdf
Generally, because of the residency test, a child of divorced or separated parents is the qualifying child of the custodial parent. However, the child will be treated as the qualifying child (for the purposes of claiming a dependency exemption and the child tax credit, but not for the earned income credit) of the noncustodial parent if all four of the following statements are true:
1. The parents:
a. are divorced or legally separated under a decree of divorce or separate maintenance,
b. are separated under a written separation agreement, or
c. lived apart at all times during the last 6 months of the year, whether or not they are or were married.
2. The child received over half of his or her support for the year from the parents.
3. The child is in the custody of one or both parents for more than half of the year.
4. The noncustodial parent attaches a Form 8332, or similar statement containing the same information required by the form, to his or her return. The form must be signed by the custodial parent. (See special rules in Publication 17 for a pre-1985 or post-1984 and pre-2009 divorce decree or separation agreement.)
See Publication 17 for additional rules for claiming an exemption for a dependent.
I would let him take you to court. You can ask for him to pay lawyers fees, if his lawyer even advises him to go through with it. If he doesn't ever have your daughter at his place, then he has no legal right to claim her on his taxes.
one note of caution. Everybody gets in a world of chaos if he were to actually file and claim her BEFORE you do. You'll both get audited when her social comes up twice. So, you need to file the VERY FIRST Day you can. That way, he is the duplicate file and he will get audited first. If you can prove he wasn't legally able to claim her (ie Tax fraud), you probably won't get audited.
Good Luck.