should I pay another 50%

I had a unsuccessful root canal procedure done on March and I paid 50% of the total cost on site based on the estimation. Due to this unsuccessful procedure, I had to have my tooth extracted. Now the insurance said, if this procedure is successful, the insurance company will cover another 50% of cost. Since it was unsuccessful, the insurance company will not cover it and the dentist sent me the bill and asked me to pay that 50% and I told I would not pay it. Today I received a letter from his attorney, can I just ignore this letter? Thanks for help!