My question involves a consumer law issue in the State of: California

I am buying my car from a former friend who, when the relationship ended badly, decided to add extra conditions to my car buying agreement that I did not agree to. He insists that I pay him for costs he incurred during our relationship before releasing the title to me, even though the amount I agreed to pay him for the car is met. I did not agree to these extra "conditions" but he insists he will take legal action against me if I do not pay them and refuses to release the title to me until I do so, and has also decided to charge me interest (which was also not in the original agreement). What action can he take against me, and how can I counter or take preventative action in order to get my car title without paying for the stuff he has added to my "bill?"