Legal Question in Family Law in Arizona

Generally, this is not correct. His obligation to reimburse the overpayment is independent of her obligation to pay ongoing support. You may be able to ask the court to reduce her monthly payment by a certain amount each month until the overpayment is reimbursed in full, but you shouldn't expect that the court will order him to reimburse you in a lump sum. You should consider offering him some sort of repayment plan that the two of you can settle on (e.g. instead of splitting the kids' medical expenses 50/50 -or by whatever proportion they currently do-, he can pay $400 toward her share of expenses, and then they can return to the original arrangement. Involving the court is probably not the most efficient way to go over a $400 dispute, but the judge will get involved if necessary.