What is the ruling on taking out a student loan based on the purchasing power of the currency at the time of repayment, and am I obliged to repent if it is impermissible, knowing that I am compelled to repay it?

The given information is insufficient to rule on the transaction, although its apparent meaning is that it is forbidden due to the stipulated increase. The criterion for fulfilling fixed debts in a certain currency is by its equivalent, not its value. However, since you proceeded with the transaction believing it to be permissible, following fatwas, there is no sin upon you. And there is no harm in you paying off the debt and the original increase associated with it, as long as you are obligated to do so.