Is it permissible to deal with the International Diamond Bank, which requests $500 and adds $150 as initial support, then sells diamonds every four days and takes 4% of the profits?

It is not permissible to engage in the aforementioned transaction, because what the bank adds to you is considered a loan that brings benefit to the lender, and that is usury (riba). The Messenger of Allah (peace be upon him) forbade combining a loan and a sale, as stated in the hadith: "It is not permissible to combine a loan and a sale." The resolution of the Islamic Fiqh Academy has stipulated the prohibition of margin trading if the broker stipulates to the client that his trading must be through him, due to it leading to the combination of a loan and brokerage. Caution must be exercised when dealing with these entities.