Is Zakat obligatory on the remaining amount from selling an apartment after settling debts and creating an investment certificate, and what is its percentage?

If you invested the remainder of the apartment's price in an investment certificate in a usurious bank, this is forbidden. You must immediately terminate the transaction. Zakat is obligatory on your capital (the remainder of the apartment's price) if a year has passed over it and it reaches the nisab. As for the usurious profits, they are illicit wealth that must be disposed of by spending them on general Muslim welfare; no zakat is due on them.

If the investment certificate is from an Islamic bank and invests in a Shariah-compliant manner, then zakat is obligatory on both the profit and the capital every lunar year. The nisab for banknotes is the equivalent of 85 grams of pure gold or 595 grams of pure silver, and one-quarter of one-tenth (2.5%) must be given.