What do these characters learn?

To you mean the banker and the lawyer.According to the narrator, the bet represents a frivolous undertaking. It is struck by the banker and the lawyer when they were both rich, in money and youth. The banker's own downfall, too, ultimately comes from him gambling his money away. Thus the story can be read as a commentary on the foolishness of gambling: it is an act derived from an obsession with material possessions, which the lawyer is ultimately able to overcome during his time in solitude. The lawyer learns, through his solitude, the frivolity of money and gambling.