The banker loses his fortune, and ends up mired in debt. But this is due to his business practices, not the terms of the bet. Years before, he'd wagered the princely sum of two million roubles that one of his dinner party guests, an idealistic young lawyer, wouldn't be able to endure voluntary captivity for fifteen years. As the years drag on it seems, to the banker's mounting horror, that the lawyer will indeed be able to win the bet after all. Since he first made the bet, the banker's fallen seriously into debt, so if the loses the wager, then he'll be ruined financially. Seized with desperation, the banker shows up at the garden house on his estate where the lawyer's imprisoned with the express intention of killing him.
Fortunately for the banker, there's no need for him to kill the lawyer. What was once a young man has now prematurely aged, mentally destroyed by fifteen years of captivity and isolation. He's come to hate everything in life, and so expresses his intention in writing to leave the house just five minutes before his self-enforced captivity is due to end. The lawyer has lost his sanity, but the banker wins the bet, and gets to keep his social status.