How Do I Resolve This Issue??

When the lease on my toyota ended in April 2010, I had mileage overage owed in the amount of $3954. Toyota/Lexus contacted me and offered a payment plan of $165/month for the next 24 months. No interest, no fees, no formal contract, just an agreement letter which they mailed me. First payment was made on 6/21/10 and all consequent payments were made in timely matter. Although this loan is current and has never been in default, I received a letter several weeks ago from a CA informing me that Toyota Motor Corp has authorized them to collect this debt. When I contacted Toyota and spoke to a supervisor, I was told that there had been a mistake, I was a great customer,and it should not have happened, but there was nothing she could do to bring the loan back in house. All she could say was, " it's out of my hands." She also said that she had her manager contact the CA to request the loan be recalled, but the agency refused. I myself called the CA to explain that they were trying too collect on a debt that wasn't in default, but the agent I spoke with had no information or paperwork what so ever about the debt. Only my name, account number & balance due. Needless to say, things heated up when I countered his threats with legal action and he hung up on me. My thought at this point is to file complaints with the FTC and state banking commission on both parties. If anyone had any thoughts or input it would be appreciated.
Thanx.