Make sauce: Bring sugar and 2 tablespoons water to a boil in a 1-quart heavy saucepan, stirring until sugar is dissolved, then boil, swirling pan occasionally, until mixture is a golden caramel, about 5 minutes. Remove from heat and carefully add vinegar, then add shallot and swirl pan over low heat until caramel is dissolved, about 1 minute.
Stir in wine and boil until reduced to about 1/4 cup, about 5 minutes. Add beef broth and boil until reduced to about 1 cup, about 8 minutes. Whisk together cornstarch and remaining 2 tablespoons water, then whisk into sauce and boil, whisking, 1 minute. Season sauce with salt and pepper and keep warm, covered.
Cook salmon: Pat salmon dry and season with salt and pepper.
Heat oil in a 12-inch nonstick skillet over moderately high heat until hot but not smoking, then saute salmon, skinned side up, 3 minutes. Turn over and saute until just cooked through, about 3 minutes more.
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