There is no clause. And Man City will only be able to sign him if he gets a work permit. Unless he becomes a Brazil regular, that likely means they would have to have him among their highest-earning players. He'd have to be bloody good for that to happen.
I don't think Luiz is good enough for them as it stands, and don't think they'll be enforcing the buy back this year.
Perhaps he is being rewarded for good games for the under-21 side? The South Americans always seem to treat the younger age group internationals very seriously.
How did he do v City?