Migration from ESA to UC

Apologies in advance if I’m posting in the wrong place. With help from this site and CAB it came to light that my son ( who has learning disabilities) had not been receiving the correct amount for ESA for 10 years. So with help as previously stated it was back dated for the ten years. I have a disregard letter from uC to state that this payment ( a considerable amount) cannot be seen as savings. Hoping this all makes sense so far. He had his usual letter from the county council saying how much his contribution would be. They do this every year going by the increase of most benefits in April. His contribution jumped from £42 to £99! I have challenged this increase. We had a meeting with said council to do a F2F assessment. Showed her the disregard letter. She said they can use this backdated payment for the past 10 years so potentially he could end up a paying them a considerable amount amount. I was under the impression that they couldn’t do this-but she insisted they can. It seems very unfair that the backdated payment that my son (I’m his appointee) received was classed as an official error from ESA but he will now be penalised as he might well have to pay back to the county council 10 years of wrong contributions! Apologies if this isn’t in the right place. I did try a search in the forum but couldn’t find anything that might relate to this issue. I hope I’ve made sense and any help that you might be able to give would very much appreciated. Maggie