Is it permissible to marry a second wife who waives her right to maintenance, and what is the ruling on this if the first wife does not consent?

The wife's maintenance is a right obligatory upon her husband. If she waives it, it is no longer obligatory for the husband. Therefore, the consent of the first wife to polygyny is not a condition, but it is recommended for the husband to appease her and conciliate her with words and money.