What is the ruling on purchasing residential units through real estate financing, where the buyer pays 20% upfront and the remainder over 20 years with a fixed installment or with a 7% annual increase?

If the intention is the possibility of choosing to purchase the housing unit for a fixed amount that does not change, then this is permissible. As for the second option, which is purchasing in installments to which a variable increase is added, this is usury (riba), which is forbidden by Islamic law. It is not permissible to stipulate any increase on the debt in exchange for delaying payment.