Who is Moshe Nachman from Tevye the Dairyman and the Railroad Stories and what is their importance?

Moshe Nachman is a wealthy Jew from a small town. He is in Yehupetz without a residence permit to see doctors. He gets the poor brother-in-law Dovid to switch identities with him so Moshe is set free and Dovid remains in jail for not having a residence permit.