do i have a winning case. took car to be repaired all verbal agreement car is being held for more money then discussed and work not completed

he has not finished the job and he want us to sing a note to pay or he will not give us our car back. we have be told that we can sue because we have to pay for a rental car and that we can get our money back for that and the police told us that we had to take this person to court in order to get the car even though they also told us that by law he would of had to write up something saying what we were going to pay and what work we was going to do and he need to call us before he did any unauthorized work.