Sprinkle sugar directly into opened can of tomatoes. Mix flour, salt, and black pepper together in a small bowl.
Heat margarine in a large skillet over medium-low heat. Sprinkle flour mixture into melted margarine, stirring constantly. Increase heat to medium-high; continue stirring flour mixture until dissolved and bubbling, 5 to 10 minutes. Add tomatoes; cook and stir until flour mixture is fully incorporated, about 5 minutes.
Stir milk, 2 to 3 tablespoons at a time, into tomato mixture; increase heat to high. Cook and stir sauce until warm and thickened, 2 to 3 minutes.