The hash is the "transaction id". Its the really long string. The second longest string is most likely your address/receiver address, the longest string is the TXID. If you sent him 0.1 BTC, he should refund you 0.1 BTC even if the product is priced in USD and the value of 0.1 BTC in USD is now different.
Go here:
https://www.blockchain.com/
Type the hash into the search box, click search and send him that link.
You are using coinbase to send funds, sometimes coinbase changes their sent address and the sent address doesn't actually belong to you. Therefore, yes, he has AN address used to send him funds but it may not belong to you. He needs a new address you get off coinbase so he can send you the BTC back.
He may need the TXID to confirm a specific job belongs to you. Though, a little surprised that people don't log transactions against users in a spreadsheet or something