Is a friend who took shares, on condition that he return the same number after a few days, obligated to pay their market value after they increased, given that he did not return the shares and missed the opportunity for them to rise?

Contemporary scholars have differed on the ruling concerning lending shares. Some have completely prohibited it, while others have permitted it. Researcher Debiayan Al-Debiayan is of the opinion that shares are a common (undivided) portion, and that lending something undivided is permissible by consensus. He also argues that a loan (Qard) is a contract of benevolence, not one of exchange, and therefore, lending shares is permissible. He further states that the borrower is obligated to return the equivalent number of shares, regardless of any change in their value.

Accordingly, your friend must return the number of shares he took from you, without regard to any increase or decrease in their value. Indeed, the procrastination of a wealthy debtor in repayment is unlawful (haram (definition)) and unjust.

To avoid such disputes, it is advisable to sell the shares for a known, deferred price to anyone who wishes to benefit his brother.