Withdrew item because it was faulty but eBay has charged fee.

[birdonawire1]

Advertised hot tub and it got lots of bids I ended it early and offered it to the highest bidder for £200. Before Collection I decided to put it up and heat it but the heater wouldn’t work. I’ve contacted the buyer, explained and cancelled the item.
eBay has charged me £20 and says I can’t withdraw this or be refunded but can have this amount deducted from future sales. I rarely sell on eBay so I will probably never get this refunded.
Can I ask them to refund me this?