the only way you'd end up with a debt is if he paid ALL he's owing for the year plus more - which sounds rather unlikely

they'd then take any over payment out of what you are entitled to get topped up from over estimating DP's income

then from your supplements (the 600 per child) and only after all of what would you end up with a debt

so that would mean your ex would have to pay you MASSES of money for it to happen!