I don't mean to trash Lee's reputation,but for the strict facts of the matter:
Robert E. Lee did not free his slaves before the war. In the first place Lee, who was a relatively poor man with a rich wife, didn't own the slaves, they were the property of his father-in-law, George Washington Park Custis, the adopted grandson George Washington, who for the most part inherited them from his grandmother Martha. Custis freed his slaves in his will but allowed his executor, Lee, to postpone their emancipation for up to five years if he judged it essential to the financial standing of the estate. Lee judged that they were and finally emancipated them at the last possible moment on December 29, 1862, more than two months after the deadline specified in the will. (Slaves of course lacked standing to sue for the enforcement of the will.) Virtually all of the slaves were behind Union lines, had been out of Lee's control since very early in the war, and were already de facto free women and men. They would have been freed, de jure, three days later by the Emancipation Proclamation
“...slaves were behind Union lines...would have been freed...by the Emancipation Proclamation...”
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Do I misunderstand what you are saying?
Slaves “behind union lines” were not freed by the emancipation proclamation.