Melt butter in a large nonstick skillet over medium-high heat. Sprinkle pork evenly with 1/2 teaspoon salt and pepper. Add pork to pan; cook 3 1/2 minutes on each side. Remove from pan.
Coat pan with cooking spray. Add shallots and garlic to pan; sauté 30 seconds. Add port and vinegar to pan; cook 30 seconds, stirring occasionally. Stir in remaining 1/4 teaspoon salt and plum preserves; cook for 30 seconds or until smooth, stirring constantly. Return pork to pan; cook 30 seconds or until desired degree of doneness, turning to coat. Sprinkle with parsley, if desired.