How is the Zakat calculated for the principal owner of a company if the company operates on an installment sale system and has partners who contribute capital in exchange for a percentage of the profit? And are the partners' funds included in his Zakat, or does he only pay Zakat on his own money, profits, and goods?

First, there is no objection to a partnership if it involves work from one party and capital from the other, or capital from both parties and work from one of them, provided that a known percentage of the profit is agreed upon for each party. It is not permissible to agree upon a percentage of the capital. It is permissible to take a monthly amount on account, provided that the profit has become apparent, and these amounts are reviewed upon final liquidation. However, if the amount paid monthly is the final profit, then this is a prohibited partnership, and it must be terminated.

Second, Zakat is obligatory upon each partner if their share reaches the nisab (minimum threshold) and a hawl (one lunar year) has passed on it. The hawl for trade begins from the ownership of the capital if it meets the nisab. The Zakat due date may differ for each partner. The nisab subject to Zakat includes liquid assets, the value of merchandise, and recoverable debts. The merchandise must be valued when the hawl is completed, and cash and debts must be added. The entire share is subject to Zakat by paying a quarter of a tenth (2.5%). The hawl can be unified by accelerating the Zakat payment for the first partner whose hawl is due, or by paying Zakat for a few months to align the hawl. Each partner may pay their Zakat themselves or authorize the company owner. The merchandise belongs to everyone and not to the company owner alone, and the partner is a partner in all that the company owns.