Can my stepdaughter's mother do this legally?

My husband pays $530 a month in child support to the state who then gives it to my stepdaughter's mother. We used to pay her directly, but she claimed we never gave her anything, and it was cash. I know, stupid. Anyway, she just called and said that we can't have his daughter this weekend unless we give her $20. Well, we don't have the $20 to give her, and she claims it is for a school field trip. But the child support is supposed to cover living expenses, clothing, and all school expenses. Can she legally not let my husband have her? The only thing that is written in the custody agreement is that he has to be up to date on child support, and he is. Plus, we don't think it's for school because she always talks about field trips coming up, and she hasn't said a word. The field trip is supposedly on Monday. There is no mention of it in her teacher's weekly newsletter, either. What can we do?