My experience has been that Saks does not give jewelry discounts except during events, i.e. Friends & Family. Even then, it's typically only 15% and Cartier is one of the brands excluded from the event.
They do offer discounts when you open a store credit card with them, but the offer varies.
As for receiving a discount at a jewelry store for a Cartier watch... I would suggest negotiating in person for better luck. A salesperson can assess your interest and decide to work with you, so when you say "This is so expensive... I don't know if this is possible or not... but is there any way that I could receive a discount on this? I really do love it... but... ", they're more inclined to want to say "yes". In the end, they can just as easily say "no". Many people have purchased the same watch - without a discount.
As for the authorized dealer comment... it's my understanding that when stores carry specific brands, the policies at the store may be a reflection of the relationship between the store and the brand. So, while a store may offer a discount with some brands, they may not be permitted to offer a discount with other brands. This happens all the time at Saks/Neiman Marcus/etc. Maybe that's what the person meant, when he refused to offer a discount (?)
HTH. Good luck!