Is the money given by a relative to a partner for investment in the company considered a debt that the partner must repay? And is the relative entitled to sue the partner with promissory notes (receipts of trust)? And does the partner guarantee the return of the capital to his relative after the company's collapse?

If the agreement between the partner and his relative is to invest the amount for profit through a legitimate Mudarabah (definition) (profit-sharing) contract, then there is no guarantee on the partner except in cases of transgression or negligence. Proving transgression or negligence is referred to experts in the field of work.