Slice the pork sausage into 1/4-inch/1/2 cm-inch slices on the bias. Cut the apple into 24 cubes. Heat the oil in a saute pan and fry the sausage slices on both sides until golden brown, remove to a platter. Wash the pan before melting the butter in the pan for the apples. Add the sugar, and fry the apple cubes, tossing occasionally, until golden on all sides. Add a little more oil to the pan, if needed, and fry the sage leaves until slightly crisp, 30 seconds to 1 minute. Lay a fried sage leaf on each sausage slice. Top with a cube of sauteed apple. Pierce with a toothpick and serve.