Dr. Salah al-Sawy of the Assembly of Muslim Jurists in America answers a question on inheritance for daughters and sons

Fatwa, posted 4.22.2010, from Qatar, in:
Religious Authority: 
Salah al-Sawy
Fatwa Question or Essay Title: 
A man has a mentally retarded son and three daughters. This man appropriated one third of his estate to his son after his death, and he did so while he was still alive. What is the ruling on what this man did?
Websites and Institutions: 
Islam Online

Children with special needs draw sympathy and pity. Feeling somehow responsible for their misfortune, their parents attempt to compensate them by favoring them in some aspects over their siblings. However, a due consideration of their needs and the special attention they deserve should not turn to be a form of partiality and unfairness.

In his answer to this question, Dr. Salah Al-Sawy, the Secretary General for the Assembly of Muslim Jurists in America (AMJA), said:

It is not permissible to bequeath a legacy to any of the legal heirs whose share is prescribed by Shari`ah. Allah has given each legitimate heir his or her share. If this was a type of gift during the father's lifetime, then the basic rule is to treat one's children equally in terms of gifts, in accordance with the saying of the Prophet (peace and blessings be upon him) in which he states: "Fear Allah and treat your children equally," unless there is some justification that would necessitate special treatment. This may very well be one of those cases considering the presence of chronic illness and disability.

In any case, the rest of the legal heirs must approve this act in order for the money to be permissible for the recipient and for the giver to be absolved of the consequences. We advise them to approve this legacy in honor of their father, out of loyalty to their brother and in hopes of pleasing their Lord the Almighty.