Is zakat obligatory on the monthly salary from which nothing remains, and on the money sent by the sister, of which the remainder varies each year? Is it obligatory for the questioner to pay the zakat on her family's money on their behalf? Is zakat obligatory on money borrowed to pay off a debt? Is the questioner permitted to pay the zakat on her sister's money deposited with her without her permission?

Zakat is not obligatory on a salary paid before the completion of a full lunar year (hawl), because among the conditions for zakat is that the wealth reaches the minimum threshold (nisab) and a full lunar year passes over it.

As for the money sent by the sister to support the family: if it is for her mother to manage, then it is her property, and zakat becomes obligatory on her if it reaches the nisab and a full lunar year passes over it. It is not permissible to give out zakat on her behalf without her permission, because zakat is obligatory upon her. If the money is for all family members as common ownership, then zakat is not obligatory on it unless each individual's share remains at the nisab at the end of the hawl.

If the number of years for which zakat became obligatory on the wealth is unknown, then one must strive and investigate to determine it and the amount of wealth involved, and pay the zakat to absolve oneself.

The nisab for money in paper currency is the equivalent of 85 grams of gold or 595 grams of silver, and the zakat rate is a quarter of a tenth (2.5%).

Zakat is obligatory on a loan (salafah) if it remains with its owner at the nisab and a full lunar year passes over it.

Zakat is obligatory on the money deposited by the sister if it reaches the nisab and a full lunar year passes over it, and it is not permissible to give out zakat from it except with her permission.

Giving charity (sadaqah) before paying zakat is permissible, but the money given as sadaqah is not counted as zakat unless zakat was intended at the time of giving it. It is permissible to pay zakat before the completion of the hawl, after the wealth has reached the nisab.