Heat the olive oil in a large skillet over medium-high heat. Add the sausage and cook 4-6 minutes, breaking up the sausage with the back of a spoon, until browned. Use a slotted spoon to transfer the sausage to a plate. Remove all but 2 tablespoons of fat from the skillet, and lower heat to medium. Add the white parts of the scallions, and cook for 1-2 minutes, until softened. Remove from heat and stir in the apple cider vinegar. Place the torn collard greens in a large bowl, and pour the warm dressing on top. Toss the greens to coat in dressing, and use your hands to massage the collards until they wilt, 2-3 minutes. Sprinkle with salt and pepper. Add the green parts of the scallions and toss to incorporate.