Is it Islamically permissible to purchase a property at an initial estimated value, with monthly installments that increase over 240 months, such that its value aligns with inflation?

It is not permissible to purchase real estate in the manner mentioned in the question, because it leads to an unknown price. This applies whether the apartments have already been constructed (installment sale), in which case it is not permissible to increase the monthly installment due to inflation; or if they are under construction (manufacturing contract), where it is stipulated that the price must be known at the time of concluding the contract. Fear of fluctuating costs does not justify making the price an estimate, unless unforeseen circumstances arise that necessitate a price adjustment by mutual agreement of the parties.