A sound structure keeps real co-ownership, leasing (إجارة), the promise, each unit sale and rent adjustment legally distinct. The objective is gradual transfer of ownership—not a loan whose return is merely renamed as rent.
The essential idea is gradual transfer of real ownership. Every payment should therefore be understood according to its true function—rent, profit share or purchase price—rather than treating all payments as one undifferentiated installment.
The sequence matters: ownership and risk change only when the relevant transaction actually occurs.