In the time between when the wager is made and when it ends, circumstances for the banker have turned for the worst. He has lost most of his fortune, and, as the deadline approaches, he realizes that this bet has the potential to ruin him. Thus, to save his own financial situation, he determines to murder his opponent. In the process, he sees the note the lawyer had left on the table, which details his renunciation of the world and his intention to forfeit the wager, rather than fall into the same material lifestyle he's come to abhor.
In the aftermath of this revelation, the banker feels a complicated mix of deep relief, remorse and self-hatred. In the end, he is financially saved and yet, in the process, his own previous mindset (and his desire to kill the lawyer rather than fulfill the terms of the bet) has validated the lawyer's condemnation by revealing the banker's own moral bankruptcy. Thus, this encounter has revealed a deeply upsetting truth about himself and his own values (and the moral corruption, this story tells us, those values represent). His sense of relief and thankfulness that he will not be financially ruined after all only makes this realization all the more self-apparent.