Well, the next half an hour within the story shows that Mr Thornhill had Miss Wilmot's fortune in his hands - and so didn't even had to marry. Perhaps it was his idea from the beginning: to bide his time, preventing the knowledge of his "marriage" (marriage after all, ha-ha!) spreading into Miss Wilmot's family, and lying to her about George's marriage until he gets money; and then let things take their course - to marry or, better still, not to marry.