It is never definitely established that the monkey's paw actually had the power to grant any wishes. Mr. White wishes for two hundred pounds and gets them as compensation for the death of his son--but this could be sheer coincidence. It might be speculated that Herbert got himself caught in the machinery that day because the family had stayed up a little later than usual entertaining a visitor, and Herbert may have had a drink or two more than usual on that occasion just because he stayed up a little later and because they were entertaining the visitor. Someone comes to the door in the middle of the night and knocks persistently. We assume that it is Herbert, or his ghost, and that he is in horrible condition--but this is something we do not know for sure. The caller stops knocking when Mr. White makes his third wish, but any caller would eventually stop knocking and leave if no one responded.
In the short story "The Monkey's Paw" the Whites are visited by a friend who tells them the story of the monkey's paw. He tells them that it has the power to grant three wishes, but that it has not been a good thing. He throws it into the fire and before it is able to burn, Mr. White removes it from the fire. He has been warned, but part of him does not believe that it will even work.
Mr. White and his wife discuss what to wish for. They wish for a certain sum of money. They find out that their only beloved son has been killed in a factory accident. A man brings them the sum to compensate for their son's death. The grieving mother wishes her son back alive. Something morbid comes banging on their door. Her husband wishes it away. Their three wishes have been fulfilled at a dire cost.