Rental champions and mastery level

Hi, so I just activated a 7 day champion that I found out that I like and would like to unlock it permenantly, is there a way to do so? When I try to unlock it, it says already owned but when I get an S it says that I need to own the champion to get it's mastery. I also have a skin shard for that champion, but since I don't "own" it, I can't use it. **Can I fix this somehow or perhaps reverse the rental?** TY~ {{sticker:vlad-salute}}
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