Getting extra money in return by the borrower by his own will

Salam! Recently I got layoff from the company. They paid me the amount of provident fund which I opted to be islamic(I demanded no interest on it). Few days after it they by their own will paid me some extra amount which as per them is the amount of interest I denied and was not willing to take. Now the company has been closed and there are no means to return that amount or ask them to take back. Is that amount halal for me or falls under Riba as there was no agreement between me and the company for that amount and there were no pre determined rates. Thank you!