Question Summary:
What if the value changes at the time of returning the loan?

Question Detail:

In a loan transaction how do you cater for time, value & money?
If I borrowed P 1,000000 in 1990 when it could buy a car is it fair to return it now when the same car costs P 1,5000000?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
 
When a person enters into any dealing with someone, he must ascertain whether it is a loan in which he wishes to assist someone or whether he wants a return.
If he intends giving a pure loan without a return, then he should clarify with his debtor the payment date of the debt. If it suits the person, i.e., he knows the person will pay on time in which he will get back the full value of his money, then he may advance a loan.
If the person is unsure or he fears the loan won't be paid on time and the value of his money will decrease, then if he cannot bear such a loss, he simply should not advance a loan. If he did that, that was his bad judgement and mistake. Now, he can not ask the debtor to increase the payment due to the loss of the value of his money due to late payment[1]. Whilst the debtor is at fault for the delayed payment, the creditor too is responsible for his wrong judgment and mistake. Furthermore, the creditor may adopt some harsh measure in recovering his debt if he fears loss of the value of his money. In brief, the concept of considering inflation in payment of debt due to delayed payments is not permissible. That will open the doors to interest. 
And Allah Ta’āla Knows Best
Zakariya Memon
Student Darul Iftaa
Chatham, Canada 
Checked and Approved by,
Mufti Ebrahim Desai.
Daruliftaa.net