Exactly as Tim has said:
The inner pad on the rear always wears down faster than the outside one and can not be viewed until the wheel is removed.
At the mileage stated, this is not premature wearing, just normal wear and tear but should have been reported at time of service.
The Dealer brake components are very expensive, but the car drove into the garage so it can drive out which would give you a bit of breathing space to source alternative parts and perhaps I can help you there as I have all the parts for the front and rear that you require at a fraction of Nissan prices.
Do not let them bully you into them fixing your car as the the choice is yours.
Just tell them to reasemble the car and return it to the condition that it was in when you brought it in.
Let me know if I can help.
Alex.