If this was a business seller and the item was returned not in the same condition as received, for example sealed packaging was opened? If not, then contact eBay Customer Services, explain you were due a full refund and only received 80%, see what they say.
To Contact Customer Services it's a Live Chat, or you can select to speak and request a Call Back. Click on this link:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
On the left sidebar you can see Have us Call you or Chat with Our Automated Assistant.
On weekdays lines open between 8 a.m. and 10 .m.
On weekends they open between 9 a.m. and 6 p.m.