I can see why you thought that.
But it isnt CMSs fault because they couldnt predict your exs response. And CMS aren't the ones with his money. You are.
You said I get child benefit. They said you've advised you are primary carer, the child benefit position supports that. You WERE entitled to money. And as long as he didnt dispute it or pursue child benefit it was accurate. We all warned you he might, and CMS wouldnt try and predict his response or offer advice on what might happen if things change.
BUT he did appeal, and they rightly said child benefit is with the mother so we believe her. But then he did submit a rival claim, and he did prove he was 50:50. If his rival claim was denied you would still get the CMS.
You cant blame CMS who didnt know your circs, and advised you based on the info they had. They couldnt have known whether hed appeal or not or how successful he would be in a rival child benefit claim. Especially as, generally speaking the odds are stacked against a father making a rival claim against the mother. But he was successful.
Now those circs have changed. And unfortunately even if thats not your fault its YOUR responsibility. You've had the benefit of the money, that you aren't entitled to because of his backdated child benefit award.
If your job over pays you, and its not your fault - you are responsible for paying it back. If your employer undertaxes you, and its not your fault - you are responsible for paying it back. If the bank accidently deposits someone else's money jn your account, and it's not your fault - youre still responsible for paying it back. Your boss, your job, your bank - they dont cover it for you. You repay it becuase you weren't entitled to it.
Hes proved you weren't entitled to the money, you took it so you owe it back. How is it fair that he doesnt get money that was taken from him, how is it fair that the taxpayer pays for money you took from him you aren't entitled to. Unfortunately the person responsible for starting the dispute, for spending the money when there was a dispute is you. So youre responsible for correcting the position. Not CMS, they dont have his money. You do.
And you can say its not fair over and over but CMS wont pay it for you. Tbh I would think it was unfair on taxpayers who fund CMS if they did pay in these kinda dispute cases.
And if you dont want to believe us that's understandable, we are internet strangers but then please take some of the many sensible suggestions of legal advice. Try citizens advice bureau even.