You have a very good question there to respond to. Obviously the central irony of this short story is the fact that when the mother begins to receive money from Paul's gains, it is obviously not enough - it is clear that she is obsessed with greed, which is one of the key themes of the story.
You will want to analyse the part of the story when Paul arranges with his Uncle Oscar to give his mother the money through a lawyer. Paul eagerly awaits to see what his mother's reaction will be to the news of this sudden windfall. However, he must have been dismayed to see the response of his mother:
As his mother read it, her face hardened and became more expressionless. Then a cold, determined look came on her mouth.
We then discover that his mother had an interview with the lawyer and demanded all the money at once. Interestingly, when Paul agrees to let her mother have the 5000 all in one go, the voices in the house are affected:
The voices in the house suddenly went mad, like a chorus of frogs on a spring evening. There were certain new furnishings, and Paul had a tutor... There were flowers in the winter, anda blossoming of the luxury Paul's mother had been used to. And yet the voices in the house, behind the sprays of mimosa and almond blossom, and from under the piles of iridescent cushions, simply trilled and screamed in a sort of ecstasy: "There must be more money! Oh-h-h; there must be more money. Oh, now, now-w! Now-w-w - there must be more money! - more than ever! More than ever!"
It is clear that satisfying his mother's financial wants does not solve the underlying problem - his mother's fixed dissatisfaction with her life and her unability to live the life she wants. Of course you will want to examine how this overriding greed affects the relationship between Paul and his mother. She is an absent parent, and this raises another of the themes of this story - her greed and desires rob her of her relationship with her son, who we can see loves her dearly, and at the end of the story, in spite of her misgivings, she chooses to leave the house for a few days, leaving Paul to embark on his final ride. Even her brother at the end of the story is sharply critical of her actions: "My God, Hester, you're eighty-odd thousand to the good, and a poor devil of a son to the bad."