It's always been a bit of a mystery to me that S.D. Lee was promoted to Lieutenant General and corps commander in the Army of Tennessee while Patrick Cleburne remained a major general and division commander.
I still cannot say why S.D. Lee got promoted that high, but I can tell you why Cleburne didn't.
Cleburne voiced an opinion after Chattanooga that slaves should be used as soldiers and offered their freedom in exchange. He said it openly at a meeting, and I think he even wrote a letter that he was ready to send to Davis if Johnston approved.
From what I understand, the only reason he wasn't carried off on a rail is that he was arguably the best division commander in the CSA, and also because Johnston and his peers nipped it in the bud. Davis knew about it, but I think Johnston shut it down before Davis ever found out. The event was suppressed so successfully, that it was not made public until 30 or so years after the war. It is rumored to have been the reason why he never commanded a corps, along with his foreign birth, which is laughable.
Ewell mentioned the same thing earlier in the war, arming slaves out of pragmatism, and he was shut down as well, but I don't think he wrote a letter, and I think it never went further than a short verbal conversation. R. E. Lee & Longstreet had to press the issue in early 1865, and even then it was nearly impossible to get congress to debate it openly.
If they had armed slaves and given them freedom in exchange, like during the American Revolution, we'd be discussing the merits of this war in very different tones. The moral high ground would not be exclusive to the North. That is precisely why some people try to promote the ridiculous idea that there were thousands of armed black Confederate Soldiers from day one, despite all the Confederate laws and and actions, such as the censure of Cleburne, as evidence to the contrary.
BTW, even when they did form a company or two of armed slaves in Richmond just weeks from the end, they were not offered freedom in exchange for it. However deluded the congressman were who believed the armed slaves would help that late in the war were, they were even more deluded to think that they would somehow be able to take veteran slaves and return them to a cotton field after a Confederate victory. Not a chance.