Is it permissible to buy shares from a friend in installments, provided that the shares remain in his portfolio and are sold upon request, with their value transferred to me, and with a purchase price set higher than the original market price?

Purchasing shares in companies is permissible under Sharia-compliant conditions, which we have explained in previous fatwas. If these conditions are met by the company, then it is permissible to buy shares from your friend at any price, whether it is above or below the market price. It is also permissible not to transfer the ownership of the shares from your friend's name, as long as you both agree that they have become your property, and your friend manages them on your behalf as an agent.