Is zakat obligatory on a commercial project that relies on debts that have not been fully paid, and how is zakat calculated from the capital, taking into consideration the madhhab of Imam Malik and others?

If all your wealth consists of debts, then there is no zakat due upon you for it, and the zakat is incumbent upon the creditors. If you repay a portion of your debts and you are left with surplus money exceeding the remaining debts, and this surplus reaches the nisab and a full year has passed over it, then one-quarter of one-tenth (2.5%) is due on it. Note that the nisab is equivalent to 85 grams of gold. As for the remaining debts, they are deducted from the wealth, and there is no zakat on them due to the incomplete ownership.