Those slaves were the property of Lee’s wife Mary until they were freed. She was Custis’ heir.
Julia Dent Grant owned those slaves and repeatedly in her own words called them “my slaves”. Don’t know how much more clear it can be.
Ron Chernow was even more clear in his autobiography of Grant on what the status of the slaves actually was. Those slaves belonged to Frederick Dent and while Julia Dent Grant had use of them she never held title. The reason for this is that Frederick Dent did not like is son-in-law and truly detested his daughter's anti-slavery father-in-law. Dent feared that if he gave ownership if the slaves to Julia then Grant, as husband, would free the slaves and there would be nothing Dent could do about it.
Those slaves were the property of Lee’s wife Mary until they were freed. She was Custis’ heir.
From the Custis Will:
“I give and bequeath to my dearly beloved daughter and only child, Mary Ann Randolph Lee, my Arlington House estate, in the county of Alexandria and State of Virginia, containing eleven hundred acres, more or less, and my mill on Four-Mile Run, in the county of Alexandria, and the lands of mine adjacent to said mill, in the counties of Alexandria and Fairfax, in the State of Virginia, the use and benefit of all just mentioned during the term of her natural life, together with my horses and carriages, furniture, pictures, and plate, during the term of her natural life”
No mention of her being given the slaves in the will.
The only place the Custis slaves are mentioned in the will
“then I give freedom to my slaves, the said slaves to be emancipated by my executors in such manner as to my executors may seem most expedient and proper, the said emancipation to be accomplished in not exceeding five years from the time of my decease.”