For marinade, combine balsamic vinegar, oil, mustard, sugar, basil, minced garlic, salt, and pepper in a small bowl. Place the leg of lamb in a large resealable plastic bag. Pour the marinade over the lamb. Seal the bag; turn to coat the lamb. Marinate in the refrigerator for at least 8 hours or up to 24 hours, turning the bag occasionally. Drain and discard the marinade.