You will pay full fees on the original listed price as eBay do not deal with partial refunds.
I presume they have paid?
If so, proceed this way.
If you are on My eBay Selling, go right of the item sold and from More Actions select Refund this item. Be careful to then add the correct amount of refund you wish to make, or you will refund the whole amount.
If you are on the Seller Hub, find the item in Orders >> Posted and Awaiting your Feedback. Tick on the little arrow left of the item from the drop down menu which appears scroll down a little, you will see Refund this item.
One tip, before you send the partial refund, mark the item as dispatched, there is a quirk in eBay's programming if you send a partial refund first, you then cannot mark as dispatched.
Use a PC / aptop and not the app, Chrome and Firefox work best with eBay.
If they haven't yet paid after purchase, come back and I'll explain what you need to do.
