Ladys Datejust 79173 needs service

[mencarl]

Hi all,

I hope you can help me with a dilemma. I recently bought my wife her first Rolex. I bought it from a trusted online dealer who offers a 2 years guaranty. we took it to our local Rolex authorized dealer to get it checked out and also to get it appraised.
Everything checked out well, but their rolex expert said that the watch was out of beat and that the mechanism needed to be serviced. They charge 600$ for doing this. The watch was supposed to have been completely serviced by the seller, but now is not keeping the time right. it's like 20 minutes behind after setting the time about a week ago.
My question to you is, should I send it back to the dealer I got it from (not Rolex authorized dealer) and have them serviced as covered by the guaranty, or should I pay the 600$ and tell the seller that he should help "cover" some of that expense?. I guess we are worried to send it back and have it not properly serviced... How difficult is this? Could not a certified Rolex expert service the mechanism and do a good job that we don't have to worry about for a while???
Thank you very much for any advice.