In Missouri, when a child support modification order is signed by a judge as retroactive, how is it credited?

We have been in a long drawn out process of modifying a support order due to extreme pay cut when my husband was laid off. He was paying his ex an amount that was ordered when he made twice his current salary. After almost 2 years, an order was signed by a judge granting the modification, and as retroactive to when the petition was supposed to take affect (but didn't b/c she disputed it, and lost her dispute in the end, we believe it was a stalling technique). Child support enforcement has not received the order yet, but tell us we will have to get the amount she was overpaid for 15 months from her. Is that accurate?? In all divorce and child support cases, the parties are not happy with each other and are probably unwilling to just hand over any money they are not forced to. So will my husband be credited the amount she was overpaid, have to ask her for it, or take her to small claims court?