What is the ruling on a son paying the expenses of his parents' household while he owes his father the price of the project of which the son is considered the owner? And is Zakat on money obligatory for him if it reaches the Nisab?

If the amount you received from your father was a loan, then he is not considered a partner in your business.

If you spent the amount, there is no zakat on it. However, if it is in your possession or used in your trade and reaches the nisab (minimum threshold) by itself or with other assets, and a full year has passed on it, then zakat is due on it.

It is not permissible for the borrower to give a gift to the lender, because "every loan that draws a benefit is usury (riba)," unless it was a common practice between them before the loan. Your spending on your needy father is obligatory, and if he is wealthy, it is recommendable as an act of righteousness and kindness, and it is not considered usury.

Please note: 1. Confirm the nature of your father's work with you; if he is volunteering, he is not entitled to payment, but if he expects compensation, then he is due his wages. 2. Be careful that your intention behind spending on him is not to waive the debt, as this would deprive you of the reward for righteousness and turn the loan into usury on your part.

Strive to be dutiful to your father, and hasten to repay the debt with sincere intention in your spending.