WTC overpayment query (after ESA award already reduced)

A brain teaser. My client was receiving WTC and Income related ESA. His ESA was reduced by the full amount of WTC. WTC was subsequently found to have been overpaid and is now being recovered from my client’s UC award. BUT my client’s grievance is that he is out of pocket if he pays back the WTC. In effect he should have had , say, £5000 in benefits of what ever type during the period, and he DID received £5000 in benefits. By paying back £4500 WTC he has lost out by that amount, and the Govt is better off by that amount. Too late to challenge WTC or ESA decisions and probably no case there anyway. Does he have any grounds to get the deductions from his UC for the overpaid WTC stopped ?

I’d agree with this previous answer, request for UC to exercise their discretion to waive recovery on grounds of no loss to the public purpose, and also financial hardship (if relevant). I would also agree it will be an uphill battle (perhaps especially as the request should have been made to HMRC previously) ... failing that, could be a debt case.