Do nothing. Keep the money. It is yours.

Here are the reasons. 1) It was your money at first place. They should give it back anyway. If not 100%, at least most of it.
2) The so called " no refund after 2 weeks rule" was set up by the university. That is not a law. They could havce set it as "double tuition if your drop out". In other words, unfair. If they had not made the mistake, you would have no way to get your money back. Since they did gave it back, it is their fault. Tell them to inprove their service, including billing. They are probablly over billing their customers( students like you,) at the first place.