There isn't really a "concession" you could give that would make Putin happy. Whatever you give Putin, he will want more. You could give them all of Ukraine, a

There isn't really a "concession" you could give that would make Putin happy. Whatever you give Putin, he will want more. You could give them all of Ukraine, and Putin would start plotting his next invasion on whatever country he wants next (Moldova, Latvia, Poland, Georgia, etc) Ultimately, He wants the "right" to expand and grow the his new Russian Empire to at least the size of the old Russian Empire or the USSR without the US or the EU or NATO interfering and getting in his way.