How did the banker and the lawyer strike the bet in "The Bet" by Anton Chekhov?

In "The Bet," the lawyer and the banker strike their "senseless" bet at a dinner party which was hosted by the banker fifteen years prior to the start of the story. The banker and his guests were discussing the death penalty and whether it is more humane to kill a man or sentence him to life imprisonment. The banker and lawyer have contrasting opinions on this topic: the banker thinks it is more humane to kill a man, while the lawyer says it is better to "live anyhow than not at all."
At this point, the banker, being a "spoilt and frivolous" man, offers the lawyer two million rubles to stay in solitary confinement for five years. The lawyer, tempted by such a high amount of money, immediately accepts the bet and extends the term of solitary confinement from five to fifteen years. Evidently, both men are keen to prove themselves right and to profit from the wager. The banker sees this as an opportunity to enhance his reputation, while the lawyer stands to profit from the bet financially. This is how the "senseless" bet comes to be.