In the short story "Dusk," the title is significant in that the setting takes place at dusk, the time between sunset and dark. The main character, Norman Gortsby, is sitting on a park bench at dusk. He is a cynical character and feels that dusk is the hour of man's defeat. In other words, people of lesser character come out at dusk.
Gortsby is right to be cynical. The second man who sits with him on the park bench in a con artist. He is trying to get money out of Gortsby. Most likely, this is the time of day when this young man comes out, all in hopes of conning someone out of his money.
When Gortsby finds the bar of soap, he finds himself embarrassed that he did not trust the young man. After learning that the bar of soap belongs to the first man who sat on the park bench, Gortsby is justified in his cynicism. He was absolutely right about the second man. Sadly enough, he has already given him money--money he will never see again.