I've met this before
Write to or see the general manager or whoever is higher than this manager, and make it clear this accusation (of scratches being there before) is unprofessional and that you feel insulted. Take photos while everyone is around and tell him you'll complaine to their managerment (if there is any), BBB, or even state Attoney General's office.
Be polite and professional, but firm!
Be polite and professional, but firm!