From Embattled Rebel: Jefferson Davis as Commander in Chief, by James M. McPherson (Penguin, 2014), Kindle Loc. 1796-1812:
On January 12, 1863, the [Confederate] president [Davis] issued a proclamation stipulating that captured officers and men of black Union regiments would be turned over to states to be tried for inciting or participating in slave insurrections. Congress enacted legislation endorsing this policy but substituting military courts for state courts. This change would have made no difference in the likely punishment—execution. But carrying out this policy proved to be impracticable. Union secretary of war Edwin M. Stanton ordered all exchanges of Confederate officers stopped so they could be held as hostages for retaliation if the Confederacy executed Northern officers. The Davis administration decided to restore captured ex-slave soldiers to bondage instead of putting them to death—though in fact many were killed by enraged Southern soldiers rather than allowed to surrender. “Captured slaves should be returned to their masters” if they could be found, Davis informed one Confederate general. “Until such time, they might be usefully employed on public works.”
On July 30, 1863, Abraham Lincoln issued an “Order of Retaliation” stating that for every Union captive executed, a Confederate prisoner should be treated likewise; for every captive reenslaved, a Confederate prisoner would be placed at hard labor on public works. This order was effective in preventing the official (but not unofficial) killing of black prisoners and their officers. But it did not completely stop reenslavement, because few Southern prisoners were remanded to hard labor in retaliation. The Confederates refused to exchange black soldiers under the exchange cartel negotiated in 1862. This refusal caused exchanges to cease, and the prisons of both sides began the descent toward overcrowding and tragic mortality that debased the last eighteen months of the war.