Overpayment recovery from UC underpayment

Really sorry if this has already been discussed, but am seeing more and more cases where a successful UC appeal has generated a large underpayment, that has then been used to recover a historic tax credit overpayment. I can’t see any appeal rights against this and was considering asking tax credits to use their discretion not to recover an overpayment, but can’t see them doing that once they have already recovered from a UC underpayment. In a recent case the addition of a disabled child premium going back 8 months (following child DLA appeal) would have generated a 4k underpayment of UC, £3500 was used to recover an old CTC overpayment that client wasnt aware of and wasnt being recovered at any rate in previous MAP’s. 
It builds in a mechanism that means underpaying UC can be in the DWP’s interest as any underpaid amount (other than for housing costs arrears) are up for grabs when the award is corrected, but it does seem to be how the system was designed. In another case there was a clear UC error in not including the correct number of children on a claim, the monthly loss of UC created genuine hardship until sorted, then the backpay was used to repay overpayment.
Does anyone have a way of challenging this recovery or is it a complete dead end?

nb - UC appeal or qualifying ben award