I have a second wife who is a teacher. I do not know anything about her salary and she does not bear any of her expenses. Is it permissible to give my first wife an allowance if she asks me in order to fulfill some of her needs?

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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

A Muslim is required to be just with his wives in terms of maintenance, clothing, housing, and so on. Nevertheless, it is permissible for him to favor some of them over the others with something when the latter are not in need of. In Al-Mughni, Ibn Qudaamah quoted Imaam Ahmad as saying regarding a man who has two wives, “He is permitted to favor one of them over the other in terms of maintenance and sexual intercourse if the other has no need of it.”

Consequently, it is permissible for the questioner to favor his first wife with some needs as long as the second has what suffices her. 

Allaah Knows best.