To some scholars, it seems that practiced Islamic banking is yet to incapsulate the ideals of Islamic economic principles. These scholars caution against mimicking conventional banking products and using the same paradigm to make these products somehow conform to Islamic legal framework.
Time value of money is the problem for the investor to avoid keeping his/her money idle and to avoid forgoing the use of money that may bring positive value to his/her investment. However, it does not mean that the investor can demand an arbitrary increase as the cost of using money without taking the risk of a productive enterprise.
Opportunity cost is a useful concept in economics. This article tries to explain how this concept is used to justify interest. Then, the article also highlights the problem with this approach.
In Mudarabah, only the Rabb-ul-Maal bears all the financial losses as the sole financial investor. If an Islamic bank enters into a Mudarabah contract as a Rabb-ul-Maal, only the Islamic bank would have to bear all the losses. Disparity in payoffs if loss occurs in Mudarabah is one of the major reasons why Mudarabah is hardly used as a mode of finance in corporate financing.
Islamic Economics Project 