Theory:

     Above the name it said ‘Urgent and Confidential’. In spite of his condition, Bepin Babu had a feeling that he ought to go through the letter. He tore open the envelope and took out the letter. This is what he read —
 
Dear Bepin,
  
     I had no idea that affluence would bring about the kind of change in you that it has done. Was it so difficult for you to help out an old friend down on his luck? I have no money, so my resources are limited. What I have is imagination, a part of which I used in retribution of your unfeeling behaviour.
  
     Well, you’ll be all right again now. A novel I’ve written is being considered by a publisher. If he likes it enough, it'll see me through the next few months.
Yours, Chunilal
Explanation:
 
Bepin's servant handed him the letter. Above the letter cover, it was mentioned, 'Urgent and Confidential'. Bepin was unwell. Moreover, he was mentally stressed with his memory issues. But, despite his ill-health, he wanted to go through Chunilal's letter. And, when he read the letter, he was shocked to see the message in it which answered all of Bepin' queries and confusions.
 
In actuality, Bepin had never been to Ranchi before, contrary to what he had believed. Chunilal was the one who set up a game with Bepin as a sort of punishment for not assisting him during a crisis. In the letter, Chunilal stated that Bepin had changed drastically since becoming wealthy. Bepin's riches let him forget about an old friend who needed a helping hand in a difficult situation. As a result, Chunilal decided to punish him and teach a lesson for his uncaring attitude. Chunilal used his imagination and the help of persons like Parimal Ghose and Dinesh Mukherji to fool Bepin into believing that he had gone to Ranchi. Finally, the mystery is solved.

In the letter to Bepin, he assured him that everything would be well and that there was nothing wrong with his health. In the letter, he also mentioned that a publisher had indicated an interest in Chunilal's novel. So, if all goes well, Chunilal would be able to overcome his troubles. The messages written in the letter were the ones described above.
  
Meaning of difficult words:
 
S.No
Words
Meaning
 1.
ConfidentialSecret that should not be told or shared with other people 
Reference:
National Council of Educational Research and Training (2008). Honeydew. Bepin Choudhury’s Lapse of Memory- Satyajit Ray (pp. 59-69). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.