Something doesn't add up here. If the contract showed unlimited mileage, on what basis did they charge you $6,400? What did they say when you called/emailed them? If you had documented proof that unlimited mileage was included then you should have paid $0. The only way I could see you coming up with a 50% settlement with them is if maybe it wasn't actually unlimited mileage and you didn't notice or something and they agreed to cut you a break. There appears to be a piece missing to this story because at least if it were me, there is no way I would have agreed to pay anything at all if I had documented proof that a mistake was made.