He bought me BTC. He sent the SEPA transfer to my Revolut account. I sent him the BTC.
Plain and simple. He scammed you.
Exactly. The buyer most probably used either others' (stolen) account details and money, either filled a complaint asking the money back (although I don't know how can that be done).
I was always afraid to sell Bitcoin directly to persons exactly because this kind of scams still happen. Sorry.
But the buyer sent a copy of his ID Card and proof of payment.