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I am sorry to hear about this situation.
It is up to you to enforce child support and child custody. If he is missing child support and is not bringing her back in time, what someone in your situation can do is file for a contempt motion. See HERE. Upon being found to be in contempt, he can be admonished, fined, made to pay for your costs in bringing the motion, or even jailed. It is at the Judge's discretion. So threatening him with such a motion or FILING it is a good way to get him to adhere to the decree.
He can file for modification of custody if he wants to. That does not mean that the Court will agree to it. The Judge makes a decision "in the best interest of the child." This includes, but is not limited to, general stability of the parent, financial stability, indoctrination of the child in the current school and environment, household condition and living condition of the child, other persons living in the house, etc. So unless he can prove that he would be better at the above than you, the Court is likely to deny him his request.
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