Answer:
The Banker
Step-by-step explanation:
The official agreement for the bet was that the lawyer would remain in confinement for 15 years straight. If the lawyer succeeded, he would win the two million dollars. However, if the lawyer left even a minute before the allotted time, the banker would win (and keep his money).
Near the end of the story, the banker reads a note/letter written by the lawyer. The note explains how, since the lawyer has altered his belief regarding the importance of material wealth and the likes, he will leave before midnight so as to lose the bet. The lawyer leaves only hours before he would have earned his prize, leaving the banker victorious.
In short, due to a change in perspective, the lawyer schemes to leave confinement shortly before his victory. He leaves just five hours early, making the banker the winner of the bet.