Partner forced to sell house by CSA who slap a bill on him 18 months later

Hi – hoping someone can advice. My other half received a court order telling him to leave his home as it was being sold to pay the 9k arrears CSA. He agreed that it was for the best and meant a fresh start etc and now nearly 18 months after the house being sold etc he has received many letters telling him he owes 9k . Obviously the sale of the house etc took time to sort out – the mortgage lenders say it’s being dealt with by their solicitors but it seems beyond harsh if he’s been forced to walk away from his home of 11 years and the CSA still want their pound of flesh .

Any ideas , advice anything would be very welcomed.