In this letter, the lawyer explains how much he has learned while in confinement. He explains that over the past fifteen years, he has learned much about a wide range of subjects. As a result, he is a completely different man than the one who voluntarily entered confinement. He has come to hate the world outside his chamber. He despises the "wisdom" of humanity, believing that humans are blind to the truth. He asserts that they "take falsehood for truth and ugliness for beauty." He now despises society and all it collectively values, and to prove his disgust toward humanity, he plans to leave minutes before his confinement is over so that he forfeits the two million rubles.
The banker reads all this and then leaves. The next morning, a watchman arrives to tell the banker that the lawyer has left. The banker goes to his chamber, retrieves the letter, and locks it away in a safe. The banker has lost most of his wealth over the fifteen years, and he realized that if he was forced to pay the lawyer the two million rubles, he would be financially ruined. As a businessman, this letter is proof that he is absolved of any financial obligation toward the man who left mere minutes before his confinement ended. His actions prove the truth in the lawyer's words; the banker is a man who values financial security over wisdom, forever hiding away the truths the lawyer has discovered, in order to protect his own position.
There is also a possibility that the banker realizes the truth in the lawyer's words and believes that if people read this letter, they will insist that he pay the lawyer the money. Although he technically didn't win, leaving just five minutes early, particularly with such noble motives, would surely cause some to look at the banker with scorn for failing to pay the lawyer. By hiding the letter, the banker prevents the world from realizing the true reason the lawyer left, thereby saving himself from a bit of public scrutiny.