G-d says to the rich man, “Not only did you fail to give of your own wealth, but also on the part that I demand be given to the poor man, you have put an ayin hara.”
As per Yitz Grossman: The rich man’s son will not have anything which the father had. The loss of wealth will be at a bad time.
What does G-d mean when he says that the wealthy man is giving an ayin hara to the very little that the poor man was given? Since the wealthy man does not receive satisfaction from his own wealth, he denigrates whatever little the poor man has, wishing to drag him down. His jealousy for the poor man’s tranquility prevents him from giving with a warm heart.
When our Sages say that the rich man’s wealth shall be lost at a bad time, they mean that just as the wealthy man is ready to buy something he greatly desires he will lose his wealth. This is a greater disappointment, since one sees his dreams shattered at the very moment they are about to be realized. Poor timing is part of the punishment. Not only will he suffer a depletion of wealth, but he will also lose it at the time when he most craves it.
Why does the punishment of the wealthy man include his son losing his wealth? Wealth in people’s eyes is often equated with happiness. Part of the satisfaction of being rich is that a person believes he has done everything possible to insure the happiness of his children. Hence, one of the greatest disappointments a wealthy man can feel is to be unable to help his children financially. This pains him even more than simply becoming impoverished, and so this shows the severity of his punishment for not giving.