In this case, the client is being required to pay for something that the client never said anything about fixing. Therefore, the client shouldn't be forced to pay the bill and the mechanic should either put the old rotor back into place or therefore let the client leave with the original payment.
If this continues into file, the client could place into a lawsuit against the mechanic for requiring a payment of something the client never was told or never said they would pay. In this case, the mechanic made a decision without the person's approval and the lack of communication should have been granted that the Mechanic never should have done anything and just told the person once they returned to pick the car up.