Is zakat obligatory on the amount of an investment sukuk that cannot be used to pay car installments, or does the mere existence of a monthly return disbursed for paying these installments make zakat obligatory on it?

First: Zakat (definition) is obligatory on money invested in a bank, even if it is held as collateral for installments, because mortgaged money is subject to Zakat if it reaches the nisab (definition) (minimum threshold).

Second: It is not permissible to invest money in the aforementioned bank, because it invests a large portion of investors' money in "treasury bills," which are usurious (riba), and in financing and facilities for clients, most of which involve impermissible organized tawarruq.

Therefore, the money must be withdrawn from the investment account in this bank. It is not permissible to use the unlawful profit to pay off car installments, nor is it permissible to keep the money in the account for this purpose. However, there is no sin in the unlawful profits taken before knowledge of their prohibition. If you need to keep the money safe, place it in a current account.