The “painful thought” that haunts the narrator in “My Lost Dollar” is that if Todd owes him a dollar and has forgotten it, it is possible, indeed it is theoretically probable, that there must be men to whom the narrator owes a dollar which he has forgotten.

The very thought “haunts” the narrator because he realises that if he starts remembering and listing all whom he owes a dollar or so, the list might get longer and longer and he might find it a real trouble to repay the debt. That is why he likes to remain forgetful of all those dollars.

Actually, the author wants to present the general human tendency in a satirical tone here that we all tend to forget what we owe others but remember vividly what others owe us.