I cannot manually refund the buyer, as explained the transaction has completely disappeared from my order history - so I cannot view the transaction, or any options associated with it (including the option to send a refund). Presumably, the transaction has disappeared from the buyer's end also - so they would not be able to open a case.

As it stands, there is no possible way to give the buyer their money back until eBay fixes this.

Thank you however for letting me know that any potential defect will be removed, if it comes to that. This is reassuring.