What is the jurisprudential adaptation (takyif fiqhi) of the issue of paying a sum of money to purchase a plot of land with the intention of concluding two subsequent contracts (a loan followed by a sale upon completion of construction), and does this fall under the category of usury (riba)?

Purchasing an apartment in installments falls under the contract of Istisna' (manufacturing contract). For its validity, the property must be described in a way that prevents ambiguity, specifying its type, size, location, and desired features. The price can be paid in installments, with a portion paid now and the remainder later.

If the contract contains ambiguity regarding the description, it is void and must be corrected.

The debt owed can be considered an installment of the price. However, if the contract is a loan conditioned on selling the property to you or manufacturing the apartment for you, it is void due to combining a loan and a sale. It must be renewed in accordance with the rules of Istisna'.

There is a difference between Istisna' (something described and to be produced) and selling what one does not own (an existing, specified item).