From my understanding, Arsenal doesn't necessarily have to sell in order to re-sign Eze ^{[they didn't sign anyone in the previous January transfer window and based on structures of the deals, I don't think they'll be paying all of the transfer fees in a single year]}... The only reason Berta is prioritizing outgoings is not to leave some of our players in limbo ^{[e.g., Viera, Nelson, Zinchenko, Lokonga and maybe Trossard (if the right offer comes)]}.
^{- IMO, Trossard has been more effective than Martinelli in the past two seasons, but because of his age & recent injuries, they might try to sell him instead.}