Project ownership issue

Hello, what happened is:
a client share a file for me to edit with “editor” rights. I edit the file, saving a local copy with a different name to not mess the original one while I try some design on it. Then when I share it with him again (both local and original file) it shows me as owner and I can only share it with “editor” rights. And even if he shares with me his original figma file I still come up as “owner”. How did it happen? and how can I make him owner again?
I tried to to the “share as: owner” but I only have “editor” and “viewer” option.

thank you!