Rude buggers, and totally unprofessional doing work on a customer's car without prior consultation. It would seem that they were hoping that you would simply agree and accept what they have done.
Based on the prices below and the fact that he didn't consult you prior to doing the job, or offer you the opportunity to source the item yourself, I would offer him $100 for the part plus 15 mins labour, provided that he can produce the one he took off and shows that it was in fact faulty. Given that there was no agreement or arrangement, be it verbal or otherwise, between yourself and the mechanic to actually undertake that task, they should be happy with the offer.
http://www.ebay.com.au/itm/A-C-Belt-...6ef3144&_uhb=1
http://www.ebay.com.au/itm/Dayco-892...797f75d&_uhb=1