Bad manangement of money
Make sure it doesn't happen again, paying 25% interest plus exchange rates on a loan is terrible money management too.
I received but I don't know from who. In 10-12 hours I have btc to refund, but to what address? He sent from exchange or personal address, what do you think?
My guess would be it's not an exchange, but it's not possible to be absolutely sure:
-the fee was very low, exchanges usually use higher fees
-the wallet used different types of SegWit addresses (starting with 3~ and bc1~) as inputs
Just don't burn the money yet until someone asks for it, and don't believe just anyone who says it's his.