Place the tofu slices on several layers of paper towels, and cover with additional paper towels. Let stand for 5 minutes, pressing down occasionally. Cut each tofu slice diagonally into 2 triangles. Lightly spoon flour into a dry measuring cup, and level with a knife. Place flour in a shallow dish, and dredge tofu triangles in flour. Heat the oil in a large nonstick skillet over medium-high heat. Add tofu; sauté 3 minutes on each side. Set aside; keep warm.
Add onion to pan, and stir-fry 4 minutes. Add the curry, salt, cumin, turmeric, and pepper, and stir-fry for 1 minute. Add tomato, cilantro, and sugar, and stir-fry 2 minutes. Add water and vinegar; bring to a boil, scraping pan to loosen browned bits. Cook 2 minutes. Spoon onion sauce over tofu. Serve with rice.