Sergeant-Major Morris does not say exactly how he got the paw, although he says he got it from the first owner.
"The first man had his three wishes. Yes," was the reply; "I don't know what the first two were, but the third was for death. That's how I got the paw."
In answer to Herbert's question, Morris says that he too has had his three wishes, so there is only one more set of wishes left. It is possible that Morris bought the paw from the first owner, or had it given to him by that unfortunate man, or even that Morris killed him in some kind of military skirmish and took the paw. It would seem that Morris is not the kind of man who would buy such a thing just on the basis of a sales pitch from a stranger. He must have had some knowledge about its powers if he bought it. Maybe he knew the first owner. Evidently he knows more about what happened to that man than he is telling the Whites.
Morris indicates that he has kept the paw because he was thinking about selling it to someone but has recently changed his mind because "it has caused enough mischief already." This suggests that he would be open to an offer by Mr. White, who ends up becoming the third owner. Mr. White is therefore the only member of the family who can make wishes. This is a good thing, because Herbert would be likely to create disaster by wishing for a million pounds or something equally extravagant. And Mrs. White, who says she would like four pairs of hands, might wish for something truly outlandish. She does in fact persuade her husband to wish for Herbert to return from out of the grave.
W. W. Jacobs was clever in selecting a mummified monkey's paw as the talisman for his story because it suggests that there might still be some vitality in the thing. Also, it suggests a far-away place like India, because there are no native monkeys in England. And furthermore, it is a loathsome object which suggests sinister potential. Mr. White doesn't want to touch it at first, but Herbert picks it up. The father is older and wiser. The son is young and impulsive. White has premonitions of danger. Herbert is a character who could get himself caught in the machinery at the textile plant where he works--especially since he will stay up later than usual, drinking more than usual with Sergeant-Major Morris, who is perceptibly a heavy whiskey-drinker.
So the reader will never know exactly how Sergeant-Major Morris acquired the monkey's paw. But this is not terribly important, because the introductory part is mainly concerned with explaining how Mr. White acquires it. What happens after Morris relinquishes control of the diabolical thing and leaves the Whites to their fate is the dramatic essence of the tale. A lot in the story depends on the unknown. For instance, the reader will never know whether what happened to the family was fate or coincidence.