Backdated child DLA = DWP taking it all for a previous over payment

our daughter was recently awarded DLA so we updated the UC journal and they replied saying we will be owed backdated DLA amount of £903. However they then said they have suddenly had a HMRC and can see that in March 2023 I was overpaid UC as my earnings were shown at the time as £989 instead of £2620. However back in March 2023 when we got the statement I actually messaged UC in the journal that I had been overpaid by UC and that they had not taken my earnings fully into account, they replied and said that they get the info from HMRC. So here I am now in 2025 ! And I owe them back roughly 1400 so they are saying they are keeping the disabled back dated amount of £903 and I will have deductions for the rest of the debt. Is this correct? We were planning to use the £903 for our daughters therapy sessions which I will now have to cancel as we cannot afford them. Sorry for the long post !!