Bring 1 inch of water to a boil in a Dutch oven. Place potatoes in a steamer basket and steam, covered, until just cooked through, about 15 minutes. Transfer to a large bowl and add arugula; cover with foil to keep warm.
Cook sausage in a medium skillet over medium heat, stirring often, until browned and heated through, about 5 minutes. Add to the potato-arugula mixture.
Remove the pan from the heat and whisk in vinegar, maple syrup and mustard, scraping up any browned bits. Gradually whisk in oil. Pour the dressing over the salad and toss until the arugula is wilted. Season with pepper.