You have already spent way too much money.
If you did not approve them to work on the rear brakes, they frankly cannot charge you for doing it. They have to approve anything you did not specifically ask them to do with you before the do it.
Take it back to the dealer, kindly tell them they worked on your brakes without your approval and now they are acting up.
Ask for them to be fixed at no charge and also ask for a refund of the work you did not approve to be done.
Either your dealer sucks, or your leaving something out.
Lastly, how come you all can't just get along?