Essentially, if the old man would not have returned to the bench to look for his soap, Norman Gortsby would not have known that the young man had deceived him. Instead, Gortsby may have begun to question his idea about the people who came out at dusk:
Dusk, to his mind, was the hour of the defeated.
Therefore, instead of Gortsy being the defeated one at the end of the story, the young man would have been defeated.
Ironically, the fact that Gortsby believes that those who come out at dusk are those who are defeated, speaks to the fact that he, himself, is defeated. Regardless, in the end, someone must be defeated because both cannot "win."
Therefore, if the old man is never seen again, Gortsby would not have known that he had been taken and defeated.
After Gortsby gives the sovereign and the cake of soap to the young man, he returns to the vicinity of the bench where he had been sitting and
...he saw an elderly gentleman poking and peering beneath it and on all sides of it, and recognized his earlier fellow occupant.
"Have you lost anything, sir?" he asked.
"Yes, sir, a cake of soap."
This of course seems to prove that the young man was a con artist and that Gortsby was a real sucker for giving him the money and the soap. What about the elderly gentleman, however? If he lost a cake of soap, why does he seem so sure that he lost it by the bench? The fact that he seems sure the soap should be there suggests that he too is a con artist and a more experienced one who has procured a cake of soap to substantiate a hard-luck story which may be the same as the one told by the young man.
If the elderly gentleman is a con artist, then his plan would have been to leave the soap near the bench, go away for a bit, and then come back to look for the soap, using it as a ploy to start a conversation with Gortsby. But when he returned, Gortsby had disappeared. The old man still wants to find his cake of soap, and the fact that he is searching for it by the bench seems to prove that he knows he had left it there and nowhere else.
There would be nothing so unusual about two con men telling the same story about losing their hotel and needing a loan to rent a room for the night. If a story is an effective one, then con artists will learn it from each other and keep using it until it gets worn out. The young con man seems like more of a novice. The old man has enough experience to know he should be able to produce a cake of soap if asked about it.
It could be that the young man was not a con artist at all but that the old man was the real con artist. The young man could really have lost his hotel and really needed a loan to rent a room for the night. Saki titles his story "Dusk" and describes that time of evening partly in order to suggest that people are shadowy, anonymous figures and it is hard to tell the good from the bad. Gortsby thinks finding the soap proves the young man was telling the truth, but discovering that the soap belongs to the old man does not prove that the young man was lying. He may have lost a cake of soap but lost it someplace else.
There seem to be multiple possibilities. (1) Both are con men. (2) Only the young man is a con man. (3) Only the old man is a con man. (4) Both are completely innocent.
Even if Gortsby never receives repayment of his sovereign in the mail, that still would not prove the young stranger was a con man. He may have rented a hotel room for the night and decided not to bother repaying his benefactor. After all, he was never going to see him again. A popular contemporary saying is: "No good deed remains unpunished." Not every loan gets repaid. The young man could justify not repaying the sovereign by reminding himself of how Gortsby had insulted him by virtually calling him a liar because he couldn't produce the cake of soap (which he really did go out to buy and really did lose).
The only way Gortsby could be assured that the young man was honest would be if he received a sovereign or a pound note in the mail.
If the elderly gentleman had not appeared at the end, the story would have been badly weakened. The reader would assume that the young man had been telling the truth and that Gortsby would get his money back in the mail.
Then the question of Gortsby being defeated wouldnt have come up, he says dusk is the hour of the defeated and that day he felt defeated, it turned out to be ture also when the old man came for the cake of soap when he realised that he was deceived by the young man who pocketed the money that he gave him and the oldman's soap too. Instead of Gortsby being the defated man at the end, the story would have ended in a very ordinary manner Gortsby blaming himself for misjudging the man, he would have even waited for the young man to come back with the borrowed money