I got an oral appliance from a dentist due to an auto accident. The appliance has never worked for me, I have been miserable with it.

I got an appliance from a dentist that does not work for me ¿ it bothers/hurts my mouth to wear it. It was paid at a very low rate through my PIP auto insurance, but they did not cash the check because the payment was so low. The dentist is hesitating my returning it, I am sure because he wants to charge me $5,600 for it. I simply cannot use it. I never signed for it when I took it out of the office. I want to simply drop it off with a witness to return it at the front desk. Can I legally do it this way? Or how should I do it?