At first, why does Mr. White have trouble making a wish on the monkey's paw?
Mr. White has trouble making a wish on the monkey's paw because:
‘I don’t know what to wish for and that’s a fact,’ he said slowly. ‘It seems to me I’ve got all I want.’
His son gives him a suggestion:
‘If you could finish paying for the house you’d be quite happy, wouldn’t you?’ Herbert said. ‘Wish for two hundred pounds, then.'
The author takes care to have Mr. White make a modest wish. Earlier Sergeant-Major Morris had cautioned him:
‘If you must wish,’ he said aggressively, ‘wish for something reasonable.'
Many of us would probably wish for something really splendid, like a million dollars. But W. W. Jacobs needed to have White wish for something "reasonable" or modest. If White wished for a million pounds, there is no way that Herbert's employers would offer to pay so much in compensation for Herbert's death. The plot depends upon the first wish being modest; and it is modest because White can't think of anything to wish for and his son suggests he wish for only two hundred pounds. The result must look as if it could have been a coincidence. Just before the man from Maw and Meggins arrives at their little bungalow, White has reminded his wife:
‘Morris said the wishes happen naturally,’ said his father, ‘so you think they’re just coincidences.’
It is a coincidence that the sum proffered as compensation for Herbert's death happens to be the same as the sum his father wished, but not a really astonishing coincidence. The man from Maw and Meggins tells the Whites that they disclaim any responsibility for the accident or any obligation to pay anything. They are really being big-hearted in parting with two hundred pounds.
In a way, the Whites are fortunate to get the money. Without Herbert as a breadwinner these old people are going to have a hard time surviving. At least they will have a house fully paid for. It is another coincidence that it was Herbert who suggested that his father wish for just enough to pay off the mortgage.