What important role did the soap play in the story Dusk?

Dusk centers on a series of interactions in Hyde park in London between Norman Gortsby (whose perspective the story follows), an old man, and a young man. Norman sees the old man sitting on the bench initially, but the man leaves before they talk. Then a young man appears, offering a story that he booked a room at a hotel, left to buy soap, and then forgot the name and location of the hotel and thus is stranded. Norman at first believes the man is lying, because he's unable to produce the soap he supposedly bought. Then, Norman finds the soap and chases the man down to give him a loan, suddenly believing his predicament and wanting to help. Finally, the old man returns and is looking for a cake of soak, suggesting that the young man may have been lying after all.
Soap is thus the central object the plot of Dusk revolves around. It's disappearance dissolves what Norman sees as the web of lies the young man is spinning, then it's coincidental return lends credibility to his story, and then the old man's return suggests that the soap in the park never had the significance Norman ascribed to it. Perhaps the author chose to use a bar of soap because of the association between cleanliness and honesty, or because soap can modify your external appearance, but appearances can be deceptive. The symbol works on a number of levels.