Let's say I have a debt of $50.
Let's say I don't pay for my credit card debts. Next month they will charge me interest upon the money I should have paid ($50 + interest).
Now let's say I pay more than I should for it, for example $75. Then Next month they will charge me -$25.
Should not they owe me $25 with the same interest they charge from me since I was the one who loaned them some money??