No retailer is authorized to give discounts by the manufacturer, period. No matter what they tell you - by giving you a discount on a certain piece - they violate the contract between them and that particular manufacturer. Many of the DF shops at the airports are in one or another way related to LVMH, most of them as subsidiaries but some directly owned by the Corp. You will never get a discount there, unless you get a really shady manager
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Of the more popular brands Omega and Tag Heuer are the strictest in this. Rolex on another hand are shown to be pretty lenient for some reason.
One of the best places to get a good price on a fine watch is in some of the Caribbean Islands. There you will get duty free price (usually 10%-20% less than the US RRP + some discount).
But keep in mind that purchasing the watch at a price lower that the RRP will auto voit your warranty with most, if not all companies. So if anything goes wrong you will have to deal with the retailer who sold you the watch rather than the manufacturer. Pretty much the same goes if you buy the watch on internet.