You have chosen two excellent stories to compare, as both focus on how some people are able to trick others into taking advantage of them in some way. What is interesting about both the man who sells the umbrella in Dahl's story and the young man in "Dusk" is the way that both have obviously carefully practised and presented their story many times to come across as convincingly as possible. The old man in Dahl's story, in spite of the natural mistrust that the mother has of strange men, spins a very convincing story, and is able to act the part, appearing to be tired and weak. In the same way, in "Dusk," the young man is able to give obvious proof of how unfortunate he is when he sits down next to Gortsby:
As if to emphasise the fact that the world went badly with him the new-comer unburdened himself of an angry and very audible expletive as he flung himself into the seat.
Both have carefully prepared and planned their "story," and both end up being successful in gaining money from their "victims," though of course it is the young man in "Dusk" who forgets a vital piece of proof to back up his story and is only able to trick Gortsby thanks to complete chance. The umbrella man too lets his story be shown to be false because of the sprightly way in which he walks away from the mother and daughter. Thus, in a sense, although both confidence tricksters have prepared their "acts" well, they let themselves down through either forgetting one vital piece of proof, or letting the act drop too quickly.