Is it permissible to deposit a sum of money in a bank and receive a monthly return from it, and what is the ruling on choosing an investment certificate with a variable return to avoid the suspicion of usury?

It is permissible to invest in variable-yield investment certificates with Islamic banks if they adhere to Sharia guidelines. This is considered a Mudarabah (partnership) in which funds are given to the bank for investment in exchange for a percentage of the profit, without any guarantee of the principal or a predetermined profit. As for investment certificates with a fixed and guaranteed return, these are considered a prohibited usurious loan. One may inquire with specialists about the best Islamic banks for investment.