What is the ruling of Islamic law regarding the tenant and their heir remaining in the leased property indefinitely at the same fixed rental value, and their arguing that "the contract is the law of the contracting parties" and that necessities permit prohibitions due to the high cost of rents and difficulty of relocation, despite our knowledge that they possess what would suffice them from this meager rent?

For the validity of a rental contract, it is a condition that the duration and the rent be known. Any law that contradicts this is considered to be against the Sharia (definition), an injustice, and an usurpation of the owner's right. It is not permissible for a Muslim to act upon it or to be content with it, even if the state authorizes it. Therefore, those who abide by a law that violates these conditions are considered usurpers and sinful. The owner of the apartment is entitled to the fair market rent for the past period. If the apartments are usurped, then prayer, eating, and sleeping in them are forbidden. As for the rule "necessities permit the forbidden," it is out of place here; the unavailability of other places for tenants or high rent is not considered a necessity that permits the usurpation of others' property.