When I first read your post that Sherman and Johnston negotiated a truce, I couldn't believe it, thinking that it was Grant and Lee who did it at Appomattox Court House.
But I looked to try to check it out in an old book, The Story of the Confederacy originally written in 1926 by Robert Selph Henry, I was surprised to find that you were correct. Sherman and Johnson did meet and talk in North Carolina, near Durham's Station. That was a week after Appomattox, and after both had gotten word of Lee's surrender to Grant. By then, they had both known as well that Lincoln was assassinated.
Wonder why it took so long for Johnston to surrender and why Sherman took it upon himself to enter into a detailed agreement with Johnston without authority from Washington (DC, that is).
Incidentally, the mobile Confederate "government" - or what was left of it - approved of the terms of the Sherman-Johnston pact. No mention in the book, though, about its terms regarding slavery.
I'm sure the Confederate government approved of it, since it explicitly allowed Confederates to retain their "property," which could have been interpreted to include slaves.
Aside from that Sherman treated Johnston and his men very well, including feeding them. Johnston never forgot. He died after standing bare-headed in the rain at Sherman's funeral.