A wife giving up her right to maintenance; and is the first wife’s approval needed before one can marry a second wife?

Maintenance is one of the rights of the wife which is an obligation upon her husband. Allaah says (interpretation of the meaning): 

“Men are the protectors and maintainers of women, because Allaah has made one of them to excel the other, and because they spend (to support them) from their means.”

[al-Nisaa’ 4:34]

If the woman foregoes this right, to which she is entitled – which is her maintenance– then it is no longer obligatory upon her husband. 

Ibn Qudaamah said: If she agrees to forego some part of her share (of her husband’s time) or her maintenance, or all of that, this is permissible. (al-Mughni, vol. 7, p. 244). 

With regard to the permission and approval of the first wife for a plural marriage, this is not a condition and the husband does not have to seek the permission of his first wife to marry a second. But it is prescribed for him to be kind to her and to spend money and to say kind words so as to calm her down and reduce her jealousy. The Standing Committee was asked about the first wife’s approval for one who wants to marry another wife. Their response was: 

It is not obligatory for the husband, if he wants to take a second wife, to have the approval of the first wife, but it is the matter of good manners and kind treatment that he should speak to her kindly in such a way as to reduce the feelings of hurt which are natural in women in such cases.  That is by smiling at her, showing that he is happy to see her, being kind, speaking nicely and by spending money on her if necessary. 

See Fataawa Islamiyyah, vol. 3, p. 204