What is the meaning of the letter?

Are you referring to the lawyer?

Yes

The lawyer wants to explain how and why he has changed over so many years. The prisoner has become entirely embittered during his captivity. He has developed an intense hatred for other humans and believes that there is nothing that he or they can do to ever reconcile this chasm. To prove his seriousness, the lawyer decides to leave his prison five hours before the appointed time, and renounces his claim to the two million, thereby freeing the banker from his debt and from financial ruin.