The best way forward would be to cancel the item, of course, with the buyer's acceptance, and relist it as a collection item .
Ensure any message to your buyer makes it very clear the reason why the item is being cancelled, that it is to be relisted on eBay again, for the buyer to buy, leave no room for doubt that this is Not an off eBay sale.
The buyer will then receive that 6 digit collection QR code for you to scan when they come to collect , and this is your only way to prove a collection item, has been collected.
The unused postage label if it previously had Buyer pays, explain to them this will be automatically refunded to them , but will take 30 days.
