Please help with homework The Last Leaf : What has the author said

On the other hand I know that I shall I remember all my life that Todd owes me a dollar. It will make no difference, I trust, to our friendship, but I shall ever be able to forget it.

• What has the author said earlier so as to make the use of ‘On the other hand’ justified?
• When did Todd borrow the dollar and why? Why does the author mention the specific date?
• What was the author’s attitude at the time of lending? What is his attitude now towards Todd?
• What special trait of his own character disturbs the author?
• Where did Todd exactly go? What did he send from there? What did the author expect to find in it?