Regular readers know that I am a deep admirer of not only President Lincoln but also Union Army General Ulysses S. Grant who also became President. Read about this exchange of letters between Grant of the North and Lee of the South. This was in 1864 when there was some political interaction regarding possibilities of peace between Lincoln and the Confederacy, but they fell through due to the South's insistence on retaining slaves. Meanwhile on the battlefield there were also negotiations:
The Confederate answer came from General Lee a few weeks later. He proposed to Grant an exchange of prisoners captured by each side in a Union attack and Confederate counterattack ten miles southeast of Richmond on September 28-30. Several black regiments participated, and some of their men were captured. Grant agreed to the exchange so long as the black soldiers were included. But Lee replied that "Negroes belonging to our citizens are not considered subject of exchange and were not included in my proposition." That was unfortunate, responded Grant, because the U.S. "Government is bound to secure to all persons received into her armies the rights due to soldiers." Lee's refusal to honor this obligation therefore "induces me to decline making the exchanges you ask." (p. 248).
From "Tried by War: Abraham Lincoln as Commander in Chief" by James M. McPherson, The Penguin Press, 2008.