As soon as one of the party people argues that a government that can't restore human life shouldn't have the right to take it away, well, we know that the theme of sacrifice is going to be important in "The Bet." Sacrifice turns out to be the most plausible way for the banker to view the actions of the lawyer—and for the lawyer himself to describe his own reaction to his voluntary imprisonment. He agrees to throw a part of his life away, to sacrifice his connection to the rest of humanity in order to find some other level of existence. But the story refuses to answer the obvious question—does he succeed?
The only person who is truly facing a great sacrifice in the story is the banker, for whom the two million large has come to mean the difference between being successful and being a complete failure.
The lawyer's final rejection of the world is totally of a piece with his adding an extra ten years to his sentence, and both sacrifices mark him as a new kind of spiritual hermit.