The chief similarity, or the common denominator, between the young con man in Saki's "Dusk" and the elderly con man in Dahl's story "The Umbrella Man" is that both have figured out ways to obtain money without working for it. The young man has invented a complicated story about losing his hotel and needing a loan to rent a room at another hotel. He gets a sovereign from Norman Gortsby. If he can do that every evening he would have an income of around three hundred pounds a year and only have to work for one or two hours around dusk. In Saki's time (1870-1916) a young single man could live very comfortably on such an income. Many clerical workers were getting a pound a week and supporting families. A sovereign was equivalent to one pound.
George Gissing, an early realist writer, describes living conditions in Victorian and Edwardian England better than any other author. His two best novels are New Grub Street (about the lives of freelance writers) and The Odd Women (about the hardships of unmarried women). As a realist, Gissing frequently mentions the gritty facts of life in London, including the price of bread and the cost of rent for a humble apartment or an attic room. He himself suffered for most of his life because he could not earn enough from his novels to enjoy any comfort or security.
The elderly gentleman in Roald Dahl's "The Umbrella Man" seems to have an independent income, but it is insufficient to support his drinking habit. On rainy days he can make enough money selling other people's umbrellas to buy all the Scotch whiskey he can hold, and fortunately there is plenty of rain in England.
Dahl's story is merely amusing, whereas Saki's story is much more serious because it touches on important social issues. Both stories are similar in suggesting the moral that you can't trust anybody. Both Norman Gortsby in "Dusk" and the young girl's mother in "The Umbrella Man" start off by being cynical and skeptical about people in general. Then when both of them let down their guards, they both discover that they were right not to trust strangers.
In Dashiell Hammett's hardboiled novel The Maltese Falcon, which is all about trust, Sam Spade has the following exchange with his lawyer Sid Wise:
Wise's fingers, running through his hair again, combed more dandruff down on his shoulders. He studied Spade's face with curious eyes and asked: "But you don't believe it?"
Spade plucked his cigarette from between his lips. "I don't believe it or disbelieve it, Sid. I don't know a damned thing about it."
A wry smile twisted the lawyer's mouth. He moved his shoulders wearily and said: "That's right--I'm selling you ou. Why don't you get an honest lawyer--one you can trust?'
"That fellow's dead."
Like The Maltese Falcon, Saki's "Dusk" and Dahl's "The Umbrella Man" are about life in the big city. Big cities are the best settings for stories, because they tend to create unique characters--or to bring out the uniqueness in characters. People need money to survive, and some city dwellers have figured out unusual ways of getting money without working for it. These schemes make stories interesting, as they do in Jim Thompson's novel The Grifters and in Arthur Conan Doyle's "The Man with the Twisted Lip."