I do not think "the dentist did not talk about the risk before the procedure" helps. However, if root canal works, you will not take the tooth out. You do not need the tooth as proof. Tell the dentist you will sue back for malpractice if he/she still goes after the remaining 50%. If he/she keeps going after you, just wait him/her in court. You have more chance to win. At worst if losing, you pay the remaining 50%.
My entist refunded me when she thought she did not do a good job on my root canal five years ago. The tooth still works ok after 5 years. Your dentist is so greedy. Do not use him/her anymore.