Hmmm, why is the middle finger with the ring? Moreover, I'm suprised that he knew about Earth's custom because I can't believe that world of chinese martial arts/cultivation novel would be with engagement rings.
@Alice I don't think he actually knew, as he placed the two rings on their middle fingers. Since the rings look like a matching pair, he probably thought it was just some kind of keepsake that would signify their connection. So he likely doesn't know that it's a marriage custom lol