Heat saute pan over medium heat add butter and sugar and heat until caramel forms. Add vinegar and plums. Cook until slightly thick. Reserve. Heat pan over high heat, slice foie gras 1/4 inch slices, and season with salt and pepper. Add to pan and cook for 1 1/2 minutes on each side. When cooked, place on a paper towel to drain. Place plums on plates and top with foie gras drizzle caramel around plate and serve.