In "Dusk," was the young man a very shrewd trickster who had actually seen the bar of soap falling out from the old man's pocket?
This is an excellent question to ask. However, in my opinion, I don't think that this is correct. I think that the young man is presented as a trickster who lacks experience and is not actually as shrewd as he thinks he is. My reason for thinking this is the way in which the young man responds when Gortsby finds him to give him some money. Note how he reacts and the way that this reaction could be interpreted:
"Lucky thing your finding it," said the youth, and then, with a catch in his voice, he blurted out a word or two of thanks and fled headlong in the direction of Knightsbridge.
The way that he "blurts" out his thanks "with a catch in his voice" and then rushes away clearly indicates that he is a young man who thought he had ruined his chance of fleecing Gortsby and now cannot believe his luck, so much so that he has to run away in order to not reveal himself. If you think about it, there was no guarantee that Gortsby would have returned to the bench and found the soap, so even if the young man had seen it, it would have made much more sense to pick it up and claim it for his own rather than run the risk of Gortsby not finding it.