How is Zakat calculated on money deposited in investment certificates for the purpose of purchasing real estate for residential and rental purposes after three years, and is Zakat due on the principal of the certificates or the return, or both, and is the principal capital measured against the real estate designated for rent that generates income?

If the investment certificate conforms to the regulations of legitimate Mudarabah (profit-sharing), then Zakat is obligatory on both the principal and the profits, because they are trade wealth. It is not possible to rule specifically on the aforementioned certificate due to not having examined its contracts. However, generally, if the legitimate regulations for Mudarabah are met, then the contract is valid, and the profits are permissible. The principal is subject to Zakat as trade goods, not as Zakat on fixed assets (mustaghallat).