This is not a free site. The way it works is that you pay upfront and then after you get an answer if you are satisfied with my service you rate me positively and then I get paid my portion of what you pay the site.
If you are not satisfied you can ask for a refund.
I suggest that you send the dentist a letter by Registered Mail. In the letter set out the facts, indicate that you came to an agreement with the collection agency and you honoured the agreement and paid what you agreed to pay. You can then say you are shocked that the dentist does not want to adhere to the terms of the agreement.
You can then say that you are most assuredly not paying anything more but that if this harassment continues you will seek legal advice about your various legal options given you are sure that this harassment is in breach of the dentist's rules of professional conduct and may if the dentist becomes persistent amount to criminal harassment.
That should work.
Or better still if you can manage it pay a lawyer in your city to send the letter.
Does that help as a starting point?
Please feel free to post back with any follow-up questions you may have. If you don't have any then I hope I have earned a 5 star rating but if you don't feel that I have please don't hesitate to reply back and let me know what more I can do to assist you. Finally, please know that even after you rate me I will be here for you and you can ask follow-up questions if you think of them later on at no further charge of course.