Contact a Fair Debt Collection Practices Act attorney. It sounds like you may have a claim under that Act, or possibly the Fair Credit Reporting Act.
If the debt is listed on your credit report, you can dispute the debt with the CREDIT BUREAU. The credit bureaus will send a response. If it is not removed from credit report, contact a consumer attorney at naca.net you can sue for damages.
The answer given is for informational purposes only and does not constitute legal advice. Dwight Bowen is a bankruptcy and consumer attorney and may be contacted at (404) 880-3310.
Frankly, I think you are making a mountain out of a molehill. It is the fact that you originally got behind on payments that has done the major damage to your credit, and this minor dispute over whether or not you owe additional money can easily be resolved by filing a dispute with the credit bureaus. The Fair Credit Reporting Act places the burden on you to correct inaccurate information before you are able to sue. I am posting a link to this law below so you can review it for yourself. Hope this perspective helps!