One difference that stands out in comparing "Dusk" and "The Umbrella Man" is that the young trickster in "Dusk" has a far more complicated story. It is hard to keep in straight in one's mind. He says he was going to stay in one hotel but found it had been torn down. So he went to a different hotel. Then he decided to go out to buy a cake of soap and have a drink, and then he realized he couldn't remember the name of his hotel and couldn't find it. What he wants is not help in finding his hotel but a loan to enable him to rent a room in yet another hotel. That is three hotels involved in his tale of woe. He structures his story in such a way that it would seem that he only needed a loan which he would pay back by mail as soon as he could get to his wallet in the second hotel, the one he lost.
In contrast, the elderly gentleman in "The Umbrella Man" is only offering to sell an expensive umbrella for one-twentieth of what it is worth. How hard should it be to sell an umbrella worth twenty pounds for one pound, especially when he is offering it to a woman who is standing unprotected in the pouring rain with her daughter? The woman who buys the umbrella is not really tricked or cheated, since she has acquired an expensive silk umbrella. The only person who has been cheated is the owner of the umbrella, who remains anonymous--but that individual has not been tricked, he has just been robbed.
Roald Dahl does a truly excellent job of describing the setting in "The Umbrella Man," especially the rain. We can practically feel the heavy rain coming down on the mother and daughter, and then feel it coming down on the elderly gentleman as he scurries to The Red Lion to get a treble whiskey and steal another umbrella.
Saki also does an excellent job of describing the setting in his story. He devotes three of his first paragraphs to describing the city at dusk, and we are made to visualize everything in a mysterious half-light. Just as rain was essential to the elderly gentleman's scam in "The Umbrella Man," so the coming on of night is essential to the young man in "Dusk." This young stranger will obviously soon be out in the cold and the dangerous darkness unless he can find someone to lend him money. The fact that it is getting so late precludes all other options. He has to do something quickly. He is a stranger in London, so he doesn't know what other resources are available to him, if any.
In both stories the "mark" is skeptical. Gortsby says:
"Of course, the weak point of your story is that you can't produce the soap."
When the young stranger claims he must have lost it, the sophisticated Gortsby says:
"To lose an hotel and a cake of soap on one afternoon suggests wilful carelessness."
In "The Umbrella Man," the young narrator's mother asks:
"If you had no money in the first place, then how did you get here?"
The Umbrella Man has a ready answer for that. He is old. He goes for a walk every day, but he always takes a taxi home because his legs won't hold up for the return journey. The fact that he is able to afford to take a taxi home every day, as he claims, enhances the plausibility that the expensive umbrella is really his. And, although the girl's mother, seems reluctant to take advantage of the old man's distress, he insists on her having the umbrella for only one pound. And furthermore, that fact that the taxi and the silk umbrella make him seem affluent suggests that his loss on the umbrella will be of no great importance to him.