medical bill dispute

I had an emergency root canal several months ago at a local dentist office. Within hours of the procedure, I incurred additional problems that forced me to seek emergency dental care which was a direct result of the poor workmanship on behalf of the primary dentist according to the 2nd dentist. I called the next day to the primary dentist and explained what happened and advised them that since I had to pay $250.00 in emergency dental care, I would be deducting it from my bill from him. I sent the primary dentist payment for the full amount less the $250.00. I made sure that there was no mis understanding about the payment and clearly wrote that this is "Payment in Full" and also wrote on the back of the check " by endorsement of this check, the endorser recognizes that this check represents payment in full". Since the primary dentist is still seeking the balance, do I have any legal rights to contest this other than going thru small claims in an attempt to relieve me of this debt I feel I do not owe?

Thanks