The financier may lease his undivided share to the client. Leasing (إجارة) an undivided share to the co-owner is accepted in the juristic discussion presented here. The client pays rent only for the financier's share that he uses, not for the portion he already owns.
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.