Honestly, we should just try to keep him. Pogba will eventually be sold for over 100m even if its not this year. We should just convince him to stay and try to win the champions league.
I mean the starting line-up would be truly scary (when Marchisio comes back in the latter stages of the champions league):
352: Buffon, Barzagli, Bonucci, Chiellini, Alves, Pjanic, Marchisio, Pogba, Dybala, Higuain.
4312: Buffon, Alves, Bonucci, Chiellini, Evra, Pjanic, Marchisio, Pogba, Pjaca, Dybala, Higuain.
We can still keep Pogba if we manage to sell Pereyra, Zaza, Hernanes and perhaps sacrifice Lemina in order for him to stay.
Herrera is perhaps the best midfielder that is attainable after he leaves and he would cost 30m anyway.