It's not possible period. In order for a player to become a free agent before his rookie contract is done, the team holding him has to decline his option. Declining that option ends the Bird Rights and makes him an unrestricted free agent. Meaning that any team could sign him and Boston would need sufficient room for a max deal. And they wouldn't be able to outbid other teams because they would no longer have Bird Rights.right but the sheer number of metaphorical blows to his head matter. Quantity has a quality all its own. Not going over the tax threshold next year would make a difference, in deferring repeater status by a year. But given paying a 3, 4, even 5x multiple on the overage, I'm not thinking about lowering AAV or anything like that, I'm thinking about having him take more before his FA hits, in order to get him for less than the max in the years when every dollar saved is more like saving $5. Doesn't sound like that's possible unless we're under the cap.
So, even if they did have the room to sign a max deal, it still makes zero sense to make him an inrestricted free agent because then every team with max cap space could bid on him and Boston would have zero advantage in any signing.