Unused Skype credit cannot be refunded because you can still use your available credit to make call from the Skype Dial Pad site on the Web:
https://calling.web.skype.com/
Just be sure to login to this site with exactly the same account as you have used with Skype.
In the case your credit has expired due to not being used within the last 180 days, you can reactivate this credit from this reactivation site.