Dental Verbal Agreement

I needed impant,post and crown done by a dentist , went for consultation, and send a pre-certified letter to the insurance. after the letter was approved, the dentist told me the OUT OF POCKET maximum amount won't be more than $600. After all the procedures completed. I ended paying $681 to the dentist, and the insurance paid their portion amount. The dentist started to ask for more money, he wants an extra of $821, I told him we had a Verbal Agreement, but he's denying that.
What can I do, any help will be appreciated. Thanks.