2008 Home buyer credit

In 2008 I took advantage of the 2008 home buyer credit and received the full $7,500.  I have filed my taxes using turbo tax since receiving this credit and have been paying it back since turbo tax said I needed to.  2020 I sold the home and turbo tax said I still owed $2500 which I paid with my taxes but the irs sent me a letter saying I still owe an additional $500.  Did TurboTax calculate wrong or did the irs?   I can’t find my old filings online that would verify I started paying this back and don’t owe any more.