What is the ruling on the landlord stipulating that the one who pays the rent first gets to rent it, if two people wish to rent it?

The validity of the option clause (khiyar al-shart) in a leasing contract (ijarah) is a matter of scholarly disagreement. The Hanafis and Malikis permit it absolutely, while the Shafi'is and Hanbalis restrict it to leases concerning a liability (ijarah fi al-dhimmah), and permit it in the case of a specified lease (ijarah mu'ayyanah) if the period is not immediately subsequent to the contract.

The most probable opinion is the validity of this condition, due to the absence of any evidence prohibiting it, and because the default rule for contracts and conditions is validity, in accordance with the hadith: "Muslims are bound by their conditions, except a condition that forbids what is lawful or permits what is unlawful." Ibn Taymiyyah held that the option clause is established in all contracts, and that conditions which do not contravene the Shariah are valid in all contracts.