Preheat oven to 400 degrees F. In a medium bowl, cream 1/4 cup of the margarine and then gradually add 1/2 cup of the brown sugar substitute, beating at medium speed with an electric mixer until it is light and fluffy. Add the egg, and then continue to mix well.
In a separate bowl combine 1 1/2 cups flour, the baking powder, 1/2 teaspoon ground allspice, and salt. Now, add this dry mix to the creamed mixture along with the skim milk, stirring just until moistened. After it has mixed, stir in the finely chopped apple.
Spoon the batter into muffin pans coated with the cooking spray, filling each about two-thirds full.
To make the topping, combine the remaining 1/4 cup brown sugar, 1/2 teaspoon ground allspice in a small bowl and then cut in the remaining 2 tablespoons margarine with a pastry blender. Mix until the batter resembles coarse meal, and then sprinkle it evenly over the top of the muffins.
Bake for roughly 20 minutes, or until theymuffins are lightly browned.