Overpayment recovery from backdated PIP

Hello,
We have a PIP claimant who should have received around £5,000 backdating of PIP (due to the time it took from claim date to decision)
He received less than expected and has been told by DWP that they have offered the Debt Management Department just over £1900 to pay off housing benefit and 2 lots of Tax Credits overpayment.

He was already paying back HB overpayment at an agreed rate and denies any knowledge of a TC overpayment (although I guess it may have been remitted - as DRH p.164). He will check with HMRC why he is thought to have a TC overpayment.

For HB - Once an agreement on overpayment recovery has already been made can the DWP change it if your finances change, especially if it is due to a backdate of another benefit?
For TC - if there is a dormant overpayment can the DWP send the backdated benefit to HMRC to pay off?

Thank you

Liz