This deal probably hasn't to do anything with whether Pogba will be sold or not. Either Mandzukic or Zaza will leave if Gabigol arrives, meaning we will pay Gabigol by selling one of them.
Not unthinkable that we already have a deal with a club for Zaza, one that the player agreed to, but we first have to replace him before the deal can be finalized. Just selling and then looking for a replacement would be too risky considering how few capable and affordable strikers are on the market.