I just had a similar situation with a car. I had a lease through Mercedes Benz credit, 3 months prior to the lease end I traded out of the car into a purchase also with Mercedes. For the next 3 months they kept sending me bills saying that I owed 1 more payment. Finally by working with the finance company I was able to get them to collect from the dealership. Your trade paperwork should be clear that they were absorbing the payoff of your trade as part of the deal. Send this to the first bank along with the dealership phone number and email.