In a large Ziploc bag, combine soy sauce, ginger, garlic, sugar, and salmon. Seal and gently shake until salmon is well coated. Refrigerate at least 15 minutes.
In a medium saucepan, bring 2 cups salted water to a boil. Cook snow peas 5 minutes, then remove with a slotted spoon and place in a bowl of cold water. Drain. Meanwhile, stir in couscous, cover, and remove from heat. Let sit 5 minutes, then fluff with a fork. Add snow peas to pot to keep warm.
In a medium skillet over medium-high, heat olive oil. Remove salmon from marinade and pat dry with paper towel. Sear until golden brown crust forms, about 5 minutes per side (watch carefully as sugar in marinade can burn). Lower heat if necessary.
Serve salmon on bed of couscous and snowpeas and garnish with scallions.