Is it permissible or impermissible to deposit retirement bonuses into bank certificates, especially if the bank is an Islamic one?

It is permissible to invest money through Islamic banks that adhere to Sharia principles, especially in variable-yield investment certificates, which resemble legitimate mudarabah. In this arrangement, the capital owner provides their money to the bank for investment in exchange for a percentage of the profit, without a guarantee of the capital or a fixed, predetermined profit. However, investment certificates with a fixed, guaranteed return are forbidden because they are considered a usurious loan. One should investigate to find the best Islamic banks that are most compliant with Sharia.