How legitimate is a partnership in which a friend invests a sum of money in a stone and marble factory in exchange for one-third of the profit for one year, and what becomes of this sum after the year ends?

The operation mentioned is a Mudarabah (profit-sharing partnership). Scholars have differed regarding the permissibility of specifying a duration for the Mudarabah contract. The majority of scholars hold that setting a duration invalidates the Mudarabah, as it restricts the agent (Mudarib). However, Abu Hanifa and the Hanbalis permitted specifying a duration, likening it to Ijarah (leasing) and agency (Wakalah). The more preponderant opinion is that it is permissible to specify a duration for Mudarabah, as long as it is with the desire of the capital owner and the consent of the other party. This is based on the general meaning of Allah's Almighty saying: (except that it be a trade by mutual consent) and the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions." Accordingly, there is no harm in giving the amount to the factory owner to operate it for one year, then returning it along with an agreed-upon portion of the profit, while adhering to the rulings of Mudarabah, which state that there is no guarantee unless there is negligence or transgression.