what do they assume about the two hundred pound they will receive as compensation

They assume the two-hundred pounds will simply appear. Herbert thinks it funny, mother believes it's probably nonsense, and Mr. White's feelings hold an ominous tone.

"I expect you'll find the cash tied up in a big bag in the middle of your bed," said Herbert, as he bade them good-night, "and something horrible squatting up on top of the wardrobe watching you as you pocket your ill-gotten gains."