Nostromo would never hear that Mrs Gould hates wealth, and say that Mrs Gould was good, and say that wealth was accursed, and then offer Mrs Gould wealth. Contrary to the surface impression, Nostromo is not, by any stretch of the imagination, speaking of the location of the silver here. He is trying to reveal the true "treasure" as opposed to wealth: love, the value of the subjective soul, what Antonia defined as "character." Note that the revelation would be to Mrs Gould "alone," though Giselle is in the room, because Giselle already knows about the primacy of love. The word "Incorruptible," applied throughout the novel to Nostromo's character as well as to the silver, also calls attention to the true meaning. It is Nostromo's climax to the dialogue, the People's message to altruism: if you really wish to be "good to the poor," do not uplift them through wealth, which is "accursed"; instead, learn their real treasure -- their inner worth -- and protect that.
Mrs Gould, of course, misunderstands.