Salaam Sardar,
Well it goes like this in my opinion. Verse 2:282, Allah commands us to get a written contract for any debt we take and list all the terms. So when one is getting a loan, they will know what the loan has in it and if it is against Riba. They will than not go through with it I would think. When a loan is taken at a fixed low rate, whether there is an inflation or not, it will not affect it.
Every bank has different rules, but most if not all that have fees, riba, interest. That is factored in the monthly payments/installments for the loan. I believe except the Islamic banks that some islamic countries have.
The best one can do is pay off as much as they can right away or try to refinance the Loan or get a bank to agree on something. I have never heard of a loan that goes towards amount one took only with no interest/finance.
If one is caught up in Riba, and did not know at the time, they just have to ask Allah for forgiveness and never repeat it again and do the best they can to get rid of the loan. I have never heard of a bank asking one to pay more than the agreement in the contract is. A pay off on the loan is something usually when a loan goes in bad standing or something that the bank agrees on due to the contract.
People do not realize that all credit cards are Riba. My God, you can sit your whole life paying it off. That is why one needs to live cash only and buy only what one can afford..
There is nothing I can find in Quran saying how it should be handled if one is in a situation as you mentioned.
Salaam