What is the ruling on selling housing units under the rent-to-own system before acquiring full ownership, where the "key to the unit" is sold and the buyer undertakes to pay the remaining installments?

For the permissibility of a lease ending in ownership (ijarah muntahiyah bi al-tamlik), it is stipulated that the contracts of sale and lease be separate, that the lease be actual, and that the guarantee and non-operational maintenance be the responsibility of the lessor, along with the application of the rulings of leasing, then selling. As for selling a dwelling before taking possession of its ownership, it is not permissible, due to the Shariah's prohibition of selling what one does not own. Naming it "key money sale" or "khulw consideration" does not change this ruling.