Trim the steak of excess fat and rub with garlic and pepper. Place the steak into a glass baking dish and pour over the vinegar. Marinate the steak for 20 to 30 minutes, turning the steak every 10 minutes. Remove the steak from the marinade and pat dry with a paper towel. Grill or broil steak to desired degree of doneness. Pour the marinade into a saucepan and simmer over medium-high heat until the liquid is syrupy. Add 2 tablespoons of cold butter to the syrup and swirl to melt the butter. Slice the steak across the grain in thin slices and serve with the balsamic sauce.