Salman Ahmed Shaikh
Opportunity cost is a useful concept in economics. Below, we try to explain how this concept is used to justify interest by some contemporary scholars.
If I have a job paying me $1,000 a month and I decide to leave it and complete my PhD for two years. Then, the opportunity cost of going to do PhD is $24,000 of job income forgone. When I am considering the option of doing PhD, I must also bear in mind this implicit opportunity cost along with fees and cost of books (out of pocket costs).
Opportunity cost of an activity is the cost of the best alternative forgone. For example, if I had another job option paying me $500 a month, then, the opportunity cost will remain to be $24,000. It is because by not doing PhD, I would have taken one of the two jobs and I will have taken the one that pays me $1,000 a month over the one that pays me $500 a month, all else equal. Then, the opportunity cost of going to PhD is $24,000 of job (best alternative of the two jobs) income forgone plus the fees and cost of books (out of pocket costs).
Just like I cannot ask or force the university to pay me $1,000 each month for me to do PhD, the owner of capital cannot ask or force the borrower to pay him/her any stipulated increase over the principal amount in a loan transaction in Islamic jurisprudence.
Time Value of Money & Islamic Finance
In investment for trade (which Islam allows), the investment goes through the entire process of a commercial activity that involves risk taking at each stage and any compensation on investment is strictly dependent upon the outcome of the commercial activity. The profit for the businessperson strictly depends upon the actual profit realized after taking entrepreneurial risk. It does not depend upon time.
Time value of money is the basis of interest. 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 lender can demand an arbitrary increase as the cost of using money.