What is the ruling of Sharia on the system of governmental housing ownership for citizens, which relies on paying 10% upfront and the remaining amount in installments over 240 months, with periodic increases in payments according to inflation and linked to the increase in employee salaries, where the title deed is received after full payment of installments or after an average of two years?

Changing the remaining amount of the item's price according to the inflation rate makes the price unknown. The knowledge of the price is a condition agreed upon for the validity of the sale. Ibn al-Mundhir said: "They unanimously agreed that whoever sells an item for an unknown, unspecified, or unquantified price, the sale is فاسد (invalid)." Ibn Abd al-Barr said: "The agreed-upon Sunnah is that the price is not permissible unless it is known." Therefore, the contract is not valid with the described condition. This is a general ruling, and the specific situation of the individual and their necessity, which may permit them to commit what is prohibited, remains to be considered.