How can the lawyer be sure of collecting if he wins?

This is a very serious bet these two men are making. The young lawyer is undertaking to spend fifteen years of his life in solitary confinement. But how can he be sure he will collect the two million rubles if he wins? It would be terrible to think that he might endure fifteen years of solitary confinement and then have the banker refuse to pay. After all, their agreement is not in writing. It is what might be called a "gentlemen's wager" confirmed and solemnized with a handshake.
The author of this ingenious story, Anton Chekhov, has taken this question into consideration and has done his best to make the bet binding. For one thing, the bet was made in front of a large number of fairly important witnesses.
The old banker was walking up and down his study and remembering how, fifteen years before, he had given a party one autumn evening. There had been many clever men there, and there had been interesting conversations....The majority of the guests, among whom were many journalists and intellectual men, disapproved of the death penalty.
Chekhov deliberately mentions that many of the guests were journalists. If the banker reneged on the bet his disgrace would be known all over Russia, because the journalists would surely see that the story got into print.
Another important factor is that the man who undertakes to serve the fifteen years in solitary confinement is frequently referred to as a lawyer. This is to suggest that if the lawyer were to win the bet and the banker were to try to get out of paying the two million rubles, the lawyer would know how to take him to court and sue him for something like "breach of promise" or "breach of a verbal contract." And the lawyer would have plenty of witnesses to call on his behalf.
Thirdly, Chekhov makes the other bettor an extremely wealthy banker.
And this wild, senseless bet was carried out! The banker, spoilt and frivolous, with millions beyond his reckoning, was delighted at the bet.
It would not be wise to make a bet involving two million rubles with anyone but a very rich man. At the time the bet was concluded the banker considered a couple of million rubles a trifle. He takes the young lawyer aside and tells him:
"Think better of it, young man, while there is still time. To me two million is a trifle, but you are losing three or four of the best years of your life. I say three or four, because you won't stay longer."
The banker fully expects to win and keep his money because he can't believe that anyone could stand to spend fifteen years in solitary confinement. But this banker finds himself in a tight spot when the lawyer amazes him by remaining a prisoner right up to the night before the time is up.
Fifteen years before, his millions had been beyond his reckoning; now he was afraid to ask himself which were greater, his debts or his assets. Desperate gambling on the Stock Exchange, wild speculation and the excitability which he could not get over even in advancing years, had by degrees led to the decline of his fortune and the proud, fearless, self-confident millionaire had become a banker of middling rank, trembling at every rise and fall in his investments.
The banker does not consider flatly refusing to pay the two million rubles. He couldn't stand the disgrace. And even if he did refuse to pay, there seems to be a strong possibility that the lawyer could take him to court and get a judgment against him for two million rubles. The banker made the bet without considering two possibilities. One was that the lawyer might actually be able to endure fifteen years in solitary confinement. The other was that he himself might lose so much money in fifteen years that two million rubles would no longer be a "trifle" but such a serious matter that he finds himself plotting to commit a murder to get out of parting with his money.