Does the loan a mother gives her son to buy an apartment fall under the ruling of usury, given her knowledge that the son will spend on her from his own money after she moves to live with him in this apartment? Is zakat obligatory on the mother for this loan? And is it permissible for the son to pay zakat on her behalf from his own money?

A mother must be fair among her children in giving gifts, based on the hadith (definition) of Nu'man ibn Bashir regarding the rejection of an unjust gift, and because a mother is like a father in being prohibited from showing preference among them.

It is impermissible to favor one child over others in giving a loan if the children are equal in need or lack thereof, unless the borrowing child is in need of the loan more than his siblings; in that case, there is no objection.

Furthermore, every loan that stipulates a benefit is forbidden, as it is usury. This does not include a son housing his mother if he intended to do so before the loan and agreed upon it with her.