"Do you have one?"
Translation:Haben Sie eines?
They would be the exact same question - except the fact that the first is casual "you" and the second is formal "you" -, only declined differently, according to the context. This question "Do you have one?" requires that you decline in the accusative form (What do you have? -I have one.). Let me put it in context to make it clear: Ich habe ein Mädchen, hast du eines? (do you have one, a girl (neuter)?) OR: Ich habe einen Hund, haben Sie auch einen? (do you also have one, a dog (masculine)?). You can clearly see here, that even in the original question, you NEED to use the accusative form there, only because of the sentence type.