Why would this be considered unethical, while not paying back would not be considered theft? If I were to lend someone some money, the deadline should be agreed upon and I would adjust it at the request of the borrower once or twice. I would also tolerate a delay of a few months, or a maybe a year, depending on the situation. But I would be pretty pissed off if they defaulted after that.
The idea that they pay back in social credit seems like a good solution to prevent this from happening again with others.