Major issue with HISD

I work part time for HISD. The other day I received an email where they said that for the past 2 years I was being paid at the wrong pay grade. Because of this they state that I owe them over 20,000 dollars. I am not a member of the teachers union and I do not know any employment attorneys. The last thing I was told is that they are working on a fair repayment plan, but in truth I am lost. Any guidance whatsoever is much appreciated.