Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
If the man in question meets all his family’s needs (expenditure and accommodation), then what he is doing is legal because he is free to dispose of his properties as he wishes. Perhaps what he did is good and recommendable because it is part of his obedience to his mother which is highly praised in Shariah.
However, if the price of this house is desperately needed to satisfy the needs of his family then it is obligatory upon him to sell this house to satisfy those very needs. If he can get the money another way he should not sell it. Allah Says (interpretation of meaning): {……but the father of the child shall bear the cost of the mother's food and clothing on a reasonable basis…..}[2:233].
There is no disobedience to his mother, because Shariah binds the obedience of parents to the Ma’aruf (things that do not contradict Shariah). The Prophet (Sallallahu Alaihi wa Sallam) said: "The obedience is in good deeds only".
Allah knows best.