Is the buyers address on the invoice in the UK?
If so, that is a customs invoice generated by the forwarding agent from the address you mailed to.
Standard practice. Your package is placed into the grey bubble bag and forwarded on. The invoice in the background is what was attached to the bubble bag. He is showing it to prove the envelope was empty when it reached the forwarding agent, not that it was later cut or inspected.
I know the buyer well, he would certainly not be attemptinh to scan for a few dollars. My best guess is that you sent an empty envelope by mistake.