What is the ruling regarding a father and his brothers refusing to marry off their daughter to a suitor who proposed to her, the reason for their refusal being to financially benefit from her monthly salary, knowing that the suitor is religious and of good character, and the girl has agreed to him?

Marriage is a provision from Allah, and a guardian is not permitted to prevent his ward from marrying a suitable man unless there is a legitimate شرعي reason. If the guardian insists on refusing, he is considered an oppressor ('aadil), and the matter should be referred to the شرعي judge to marry her off, or order the more distant guardian to marry her off, or appoint someone to marry her off. One should not despair of trying to convince the guardian, and wishing for death is prohibited except for a valid purpose. The financial support of poor parents is obligatory upon their children.