Sage is a very powerful flavor, just as brown butter is as well. Honestly, I'd be a little afraid of the sauvignon blanc being dwarfed by the food, depending on what style of sauvignon blanc it is. Sage has a tendency to leave a distinct aftertaste that might put the flavors of a white off balance. I'd probably pair this with an earthy Barbera and try to play strength against strength..

Well there are two theories of food and wine pairing which are essentially contrast or complement. The core tenet of each is to match a wine of like intensity to the food. I could see certain Sauvignon Blancs being a decent match for this dish as there is the complementary green aspect but the wine may have little more to offer.

I would approach this in one of 2 ways. A lighter herbal red such as Schiava or a Cabernet Franc from The Loire valley would have the requisite acidity to stand up to the butter and in addition the the herbal element should have nice berry fruit to parry the brown butter. Alternately an aged Syrah, preferably from the Rhone, would have an olive element which could draw contrasting tones from the sage and an earthiness that would pair well with the potato and brown butter.

Honestly much of food pairing is so very personal that i wouldn't worry too too much about it. If I were having the dish i would opt for either the Schiava or a young Nebbiolo or Barbera. They would certainly work with the dish and they are the wines a generally drink!

Let us know what you go with and how it worked out!