This is why it's a bad idea to borrow money from family members and friends. Can you be charged interest? I don't see how - from what you are saying, there are no repayment terms. There is no agreed on interest rate, etc. etc. On the other hand, your fiance got into a bad situation where she agreed to pay a rent of $900, but can't. So the mother (very generously it appears) lowered that payment to $500. I would tack on a monthly interest charge too, if I were the mother and had cut your payments in half already.

So then your fiance turns around and trades in a car (worth $3,000) and apparently agrees to purchase a newer vehicle. So your fiance couldn't pay the rent, was given a generous discount, and then turned around and traded in the car she was given (and likely incurred car payments) while disregarding the money owed to mom.

This isn't a legal question. This is a question about "doing the right thing". It sounds to me like your fiance's mom took care of her daughter and helped her out when she needed it. Do the right thing and be thankful for the help. Pay mom back.