It says in the article that "his will was submitted to the Alexandria County Court for probate on the first day of its session (7 December) after arrival of the executor at Arlington and is there on record in his own handwriting, open to inspection". So, it seems there is access to the will as it was written.
The question for me then becomes whether a verbal expression is sufficient to be binding.
"An oral will, which is also called a nuncupative or deathbed will, is a will that is spoken to witnesses, but not written. Such wills are valid only in a few states and only in very limited and unusual circumstances. The idea is that if someone suddenly becomes ill or in extreme danger, and can’t make a written will, his or her last wishes will be honored.
Is It Legal?
If someone dies without leaving a valid written will, witnesses who heard the deceased person’s last wishes about his or her property may come forward and claim that the person made a valid oral will. It’s an uphill battle, however, to prove that a deceased person’s last words constitute a legally binding will."
Captain Robert E. Lee comments on his father's oversight of business matters:
"One marked characteristic of my father was his habit of attending to all business matters promptly. He was never idle, and what he had to do he performed with care and precision. Mr. Custis, my grandfather, had made him executor of his will, wherein it was directed that all the slaves belonging to the estate should be set free after the expiration of so many years. The time had now arrived, and notwithstanding the exacting duties of his position, the care of his suffering soldiers, and his anxiety about their future, immediate and distant, he proceeded according to the law of the land to carry out the provisions of the will, and had delivered to every one of the servants, where it was possible, their manumission papers. From his letters written at this time I give a few extracts bearing on this subject:
“...As regards the liberation of the people, I wish to progress in it as far as I can. Those hired in Richmond can still find employment there if they choose. Those in the country can do the same or remain on the farms. I hope they will all do well and behave themselves. I should like, if I could, to attend to their wants and see them placed to the best advantage. But that is impossible. All that choose can leave the State before the war closes....
“...I executed the deed of manumission sent me by Mr. Caskie, and returned it to him. I perceived that John Sawyer and James’s names, among the Arlington people, had been omitted, and inserted them. I fear there are others among the White House lot which I did not discover. As to the attacks of the Northern papers, I do not mind them, and do not think it wise to make the publication you suggest. If all the names of the people at Arlington and on the Pamunkey are not embraced in this deed I have executed, I should like a supplementary deed to be drawn up, containing all those omitted. They are entitled to their freedom and I wish to give it to them. Those that have been carried away, I hope are free and happy; I cannot get their papers to them, and they do not require them. I will give them if they ever call for them. It will be useless to ask their restitution to manumit them....”
Recollections and Letters of Robert E. Lee by Captain Robert E. Lee, his son
If we were to look exclusively at the legal issues involved, it seems that Robert E. Lee followed the letter of the law in the circumstances, at least as they were permissible at the time. It doesn't make it any more palatable, and it may be possible to fault him in other matters, but the OP specifically references the will of which he was Executor, so that is what I am focusing on here.
Thanks for the background.Considering the political bent of the Traveller, one might suspect that they had, let us say, an opposing and "peculiar" view as to slavery and abolition. And were not above throwing shade at a slaveholder, esp. a prominent one such as Lee, by "stretching" the truth. From Wiki:
Compared to other papers in Boston in the 1840s, the Traveller was notable for its significantly lower retail price, and for being sold on the street. It supported the views of the Free Soil Party and the Republican Party. Its office was at no.31 State Street (c.1851–1894).