Is it permissible to give a vow to needy brothers and sisters, and is it obligatory to give the salary increase if the vow was to donate a month's salary, and does the vow become void if the salary is no longer sufficient for one's needs?

A vow conditional on a disliked act is nevertheless obligatory to fulfill by consensus. It is permissible to give the vowed money to two poor brothers if no specific recipient was designated for the vow, as they have a greater right to it. However, if a specific recipient was designated, it is not permissible to give it to anyone else. If you vowed to donate only one month's salary, your obligation is fulfilled by donating one month's salary, and you are not obligated by any subsequent salary increase. If you vowed to donate one month's salary every year, then the salary increase follows the original vow. If you are unable to fulfill the vow, it remains a debt upon you. If the inability is total and permanent, you must offer an expiation for an oath. If you are able to fulfill part of the vow but not the other part, then donate what you can and offer an expiation for an oath for the part you were unable to fulfill.