Nervous. Sold Refunded Pixel On Ebay Now User Is Asking For Receipt Because Google

I'm nervous.. I sold a refunded pixel on ebay, and now the user is requesting a receipt. He said that sometimes google asks for it for warrantee purposes. I told him that I didn't have one, and suggested that he send the phone back to me for a refund. But he may not want to do that, so I'm really concerned that he's going to call google, in which case I'll be charged with fraud.