Do I have to pay the mechanic for the unauthorized work?

I arranged to have a mechanic replace my transmission, he quoted a price of $3600.00, I said ok. I told him if it cost any more than that I needed to be contacted. I was in contact with him on a daily basis while repairs were being done. When the work was finished he told me I could pick it up in the afternoon and the cost was $3500.00 (great right?) I told him that I would be at work in the afternoon and I would pick it up the next morning. When I got to the shop the mechanic told me he also put new brakes on and wanted another $300.00. I knew I needed work done on the brakes but had planned to have a friend do them. I did not authorize them to do work other than the transmission. I feel they took unfair advantage. I have already paid for the transmission.