Is it permissible for the testator to favor one of his children in inheritance due to an excuse such as illness or otherwise?

The questioner : It is well known among some of the common people that it is permissible for a person to make a bequest, even to some of the heirs, if that heir is in need of continuing his studies or is a patient in need of permanent treatment, or something similar from the necessities. We ask you to kindly clarify the ruling of the Shari'ah in this matter.

The Shaykh : It is not hidden that a bequest concerns the testator regarding something after his death. Therefore, I do not believe that the reasons you mentioned justify, according to the Shari'ah, making a bequest to someone in the state of illness or condition you described, except in one case: if the illness is one from which recovery is not expected and it actually requires assistance for the patient's treatment. In such a case, I do not see any harm in making a bequest for treatment. Otherwise, the patient may recover and the healthy one may fall ill, as he (peace and blessings be upon him) said in the authentic hadith: Therefore, it is not permissible to make a bequest out of precaution, because this patient may recover. However, if his illness is of the type I mentioned earlier, which is an illness from which recovery is not expected, and there is not in the inheritance—this is a second condition—enough of the inheritance that he will be entitled to upon the testator's death to cover his treatment, then if we observe these two conditions, the bequest is permissible. Otherwise, it is an unjust bequest and is invalid.
Abu Malik: The hadith says: "The healthy one may fall ill, and the lost one may be found, and the animal may be lost."The Shaykh : ... The lost one is considered based on what may happen, not based on what is currently the case.