Aibu to refuse to return child maintenance to ExH

[meikyo]

Ex H just emailed me to demand that I pay him back over £500 in child maintenance. He is on benefits (hasn't worked for most of DD's life - she is 13) and has been paying £30 a month for 17 months. We split up 8 years ago and I am v happily remarried.
CSA/CMA haven't been involved for about 4 years, I have a letter from them dated 2012 formally closing the case. So in last few years Ex H was paying voluntarily.
Ex H has DD EOW one night and also has her during some holidays.
He says in the email that he contacted CMA for advice about overnights and on the basis that he has DD over 52 nights pa he should pay zero.
I accept this is the case for CMA deductions. However, I don't agree with his calculation of overnights as he has made certain assumptions which I can refute with actual dates etc and in any case, he was paying voluntarily with no CMA involvement.
His email is aggressive and confrontational. He has form for this and recently took me to family court on a spurious case which the Sheriff threw out quite forcefully.
I responded to his email by telling him it is DD'S money and he paid it voluntarily. Given his form I'm expecting he will try legal action to recover the money.

I don't know a huge amount about this but my sister is in the process if going through CMA as her ex husband often reduces the amount they agreed. When she spoke with CMA they told her that as it us nit a legal agreement, I've via them, there is nothing she can do. Therefore if your current agreement is a private one he has no legal right to demand you pay him back. Hope this helps

Can you reply with (but obviously not do..)

Yeah, sure. I'll just make dd throw up so you can sell the food she had eaten to get some money back. And I'll fish those old shoes out of the bin now are too small. That'll get you another couple of quid.

I think there must be a chocolate wrapper in her bin. I'll grab that too. Oh, and the hairdresser sweeping from her last haircut.

There's definitely some ash left in the bbq, from the bbq she asked for last weekend, and half a burger shoved in the back of the fridge.

Anything along these lines, pointing out that the money has been spent on his dd...

Tell him to fuck off but no matter what you do don't expect him to carry on paying because now he has been effectively given permision to stop

Don't pay anything back and don't take his word for it that he is telling the truth about what CMO/CSA said he has to pay.
Even though he is on benefits he is still usually expected to pay a small proportion of his income towards the upkeep of his child. He would only be exempt if the overnight contact was 50/50.