1. Is it permissible to send daughters to western universities in Europe and the U.S. for study?
2. If one person has four sons, but not all the sons care for the parents equally. One even does not give money to the parents, but other give. Can father give his most obedient son some amount from his earnings or saving during his lifetime as a special reward?
3. A man has only one daughter who is married and three sons who are married and well-off. Can the father give his daughter 10 or 20 percent of his wealth to his daughter; the rest of his wealth will be left to be shared among his heirs (the same daughter, three sons and wife) after his death?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The answer to your first question is that it is not permissible for a Muslim to send his daughters to study in non-Muslim countries, because this leads to many disadvantages. Among these disadvantages is that they might travel without being accompanied by a Mahram, and they might be subjected to reasons of temptation there because of the dissoluteness and immorality that is widespread in mixed universities. Therefore, the guardian of these girls should be very careful so that he will not regret when regret is of no avail.

For more benefit, please refer to Fataawa 82730, 82431, 81846 and 84711.

As regards your second and third questions, we say that it is an obligation on the parents to be just between their children when gifting them according to the most preponderant opinion of the scholars because the Prophet said: "Fear Allaah and be just between your children (when gifting them)." [Al-Bukhari and Muslim] However, some scholars are of the view that it is permissible to prefer a child over another in gift giving if there is a sound reason for favoring him over his brothers and sisters, like poverty, sickness, or dutifulness and serving parents. Therefore, it is permissible to favor this son who used to financially help his father, and it is also permissible to favor that girl if for instance she needs help and her husband is poor and he has no money to spend on her.

For more benefit, please refer to Fataawa 92564 and 89862. 

Allaah Knows best.