Serve this German-style side dish with sausage, chicken cutlets, or roasted meat.
Bring vinegar and sugar to a boil in a small saucepan. Reduce heat; simmer, stirring occasionally, until liquid has reduced to 2 tablespoons, 5 to 10 minutes. Add butter, and swirl until melted. Transfer to a medium bowl.
Cook beans in a pot of boiling salted water until tender, about 6 minutes; drain well, shaking off any excess water. Toss with vinegar mixture; season with salt and pepper. Serve immediately.