I understood the situation differently. Mr. Thornhill had Miss Wilmot's fortune because the papers were already signed in anticipation of a formal marriage ceremony. I do not think the fortune was independent of it. If he refused to marry her, then I am not confident that he would be able to keep the money. That would be breach of promise. But if Miss Wilmot refused, then the situation would be different and he could keep the money if he chose. I suppose that the Wilmot's could have tied up the money for a long time in the courts if they tried to fight it, claiming Thornhill deceived her (e.g., about George's marriage, marriages to others, etc.). But fighting for the truth on moral grounds doesn't seem to play a large part in this story.