It depends on the team's cap situation. Say he wanted to come to Phoenix, which I'm only using as an example, then we could sign him to the most money we have available right now for 1 year and then work to free up more space to give him a max deal next summer. I'm not sure how our salary situation looks next summer but with Tyler Johnson's contract expiring, we might have some more available if we don't spend elsewhere this year. Again, I'm only using the Suns as an example. If the team he signs with can clear max space for next summer then he'd be free to sign now for 1 year and getting a max next summer but that team couldn't go over the cap to sign him because they wouldn't have any bird rights with him.
However if Durant wants to go to a team without max room now, he could ask Golden State to facilitate a sign and trade because they have his bird rights they could sign him to a max and trade him but to do that they need to take back salary. That's up to Golden State as far as if they want anything or not. They could do a sign and trade with New York and accept a pick in return because New York has the cap space to absorb his max deal but if he were traded to us in a sign and trade, we'd need to send back enough to clear space for him which would probably mean Warren and Tyler Johnson, plus the rest of our cap space. They would get a small trade exception for the difference in salary they sent out, KD's max, and what they take back, whatever is between KD's salary and TJ and TJ's contract together. So KD still gets paid regardless, it's just a matter of the Warriors playing ball and KD wanting to go to a team that can't sign him outright, which is unlikely.