Ruling on employment without a fixed salary

This sister asking from Yemen says: I have a son who is employed by a wealthy man, and he does not have a fixed monthly salary; because whenever he needs money, the man gives it to him. If we estimate what is with the merchant, we find it to be about one hundred thousand riyals, but in the eyes of the wealthy man, he considers it part of his capital which he trades with. That is to say, he does not give his son money; rather, he includes what is his right within his own capital, i.e., the merchant's capital. And when it came to paying zakah, my son said to the merchant: How much do I have with you so that I can pay the zakah on my money? The merchant said: I am the one who will pay zakah on your money along with my money?
Answer: This worker must be given a fixed salary, and it is not permissible for him to work like this without a fixed salary; because this leads to dispute and litigation, and because it is an unknown salary and an unknown wage, it is not permissible. The obligatory thing is that they agree on a fixed salary for him, and after that, he is the one who pays its zakah whenever a lunar year passes over it; his employer, the worker, pays its zakah. And if they agree on a known amount for what has passed, then the worker pays its zakah after a lunar year has passed over it. Yes.
The presenter: May Allah reward you.