The difference between the value of the currency at the time of payment and the time of borrowing does not warrant asking for the mentioned increase, for it is a type of the unlawful usury. It is obligatory to repay what he borrowed without the extra amount which the lender asked for. As for the difference in the currency value, it is predestined by Allah. The value of money may increase to equal the initial amount borrowed, or even more, because rates are variable. This ruling also applies to what you have mentioned in the case of the car. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.