What is the ruling of Islamic law on the "sale on credit" contract for purchasing property in Europe, which includes setting an initial price with a down payment and monthly installments linked to a cost-of-living index (with annual increase or decrease), and the seller retaining the usufruct of the property until their death or for a specified period, with the buyer gaining full ownership of the property at the end of the term? Is this type of contract considered gharar (excessive uncertainty) or a prohibited risk? What are the necessary Sharia and legal conditions for the validity of this agreement?

It is permissible to sell on credit or in installments at a price higher than the spot price. It is also permissible for the seller to stipulate a known period for benefiting from the sold item. However, it is not permissible if the period is unknown or linked to the term of death. Similarly, it is not permissible to link installments to an index that leads to an unknown price at the time of the contract, due to the resulting gharar (definition) (excessive uncertainty) and riba (usury). Based on this, it is not permissible to purchase the property in the manner mentioned, as it includes stipulating an unknown period and linking installments to an index.