I think we are speculating quite a lot here
Yes he is a good player but Chelsea spend so much money whenever they want-I don't think they are linked to this deal.
He will go to France for free and they will probably keep him, talent develop him (similar to how Chelsea operate) and sell him on eventually for a larger fee, maybe back to man city after a couple of years.
I don't think Chelsea will take him on loan or after a season to avoid paying a small compensation that is quite unlikely based on Chelsea's track record