It should be noted first of all that dutifulness to parents is obligatory according to the Qur’an and the Sunnah of Prophet Muhammad (peace and blessings be upon him). It deserves great reward in one’s life and after death. With regard to the question, it is legally asserted that so long as a person is of a sound mind and body, it is permissible for him or her to spend their money or to dispose of their properties in allowable ways.
In his response to your question, Dr. `Ali Jum`ah Muhammad, the mufti of Egypt, said,
It is permissible for people to dispose of their property during their lifetime the way they like, but this is subject to the following conditions:
1. A person must be in a good health.
2. A person must spend his or her money in a lawful matter.
Thus, the contract is valid if it was concluded during the mother’s lifetime, and if it fulfilled the above-mentioned conditions, regardless of the fact that the son did not pay for the apartment.
It is permissible for the seller to absolve the buyer from paying, making the sold item a gift. This is based on the opinion that states that the important factor in a contract is the intentions and meanings, not expressions and terms. According to this opinion, the apartment is a gift, though it seems to be a sale.
The apartment is not part of the mother’s estate. It belongs to the person in whose name it is written, and no one is entitled to ask for a share in it.
May Allah guide all of us to what pleases Him!