Clean and disjoint the duck. Dip the duck into soy sauce.
Over high heat, cover the bottom of a heavy skillet with peanut oil to about 1/2 inch deep.
When the oil is hot ("sizzling") add the pieces of duck and brown it evenly on both sides. Remove duck from skillet and drain on paper towels, pour out extra fat from pan, then replace browned duck into pan. Lower temperature to medium low.
Mix the remainder of the soy sauce with the mashed canned plums, reserved plum syrup, ginger, chopped green onion, garlic, sweet sauce (if using), and salt. Pour the mixture over the browned duck pieces in the pan, cover, and let cook over slow heat for 30 minutes or until the duck is tender.
Mix together the cornstarch with 2 tablespoons plum juice and stir until smooth.
Remove the duck and thicken the sauce in the pan over low heat by gradually stirring in the cornstarch mixture. Cook until smooth and clear.
Pour the sauce over the duck and serve.
Re "Chinese sweet sauce": the recipe nor book do not give any further info on the "sweet sauce" used, but I imagine that such substitutions could be made as an Asian based sweet duck sauce, plum sauce, or simply a tablespoon of brown sugar for each tablespoon sweet sauce.
Note: in these health conscious days I would probably not use peanut oil to brown the duck, but rather olive oil. If I did use peanut oil, I certainly would not use 1/2 cup of it, as duck is a meat which is quite full of fat to begin with. It's up to you how you'd like to brown your duck and what amount of oil to use, but I've presented the recipe as originally given in the cookbook.