Pull a Morata on the buyer, insert a buyback clause; first-refusal option.
If selling Zaza could help retain Morata, then obviously go for it, but only after the Euros, I doubt his price would fall much if he were to underperform. (provided he is selected to go). However, should he perform remarkably it would mean $$$.
If Real Madrid are adamant on bringing back Morata, then keep him.
Honestly, it all depends on Allegri's future plans for the squad too. Berardi coming would mean we'd have two SS (Dybala, Berardi), 1 target striker (Mandzukic), Morata, & Zaza. Obviously one must leave: Morata over Zaza to stay.