They were his father-in-law's slaves. In his will, George Washington Parke Custis stipulated that all the Arlington slaves should be freed upon his death if the estate was found to be in good financial standing or within five years otherwise. However, upon his death Custis left a good number of debts that still needed to be paid off, which resulted in the slaves not being freed immediately per the will. This was done in order to pay off his father-in-law's debts, not out of a desire to own slaves. In a letter to President Pierce, Lee wrote that "There are few, I believe, in this enlightened age, who will not acknowledge that slavery as an institution is a moral and political evil."
You are correct that Grant freed the slave that was under his name in 1859. However the family owned four others under his wife's name, although Grant himself was responsible for supervising them. That being said, of course, in that day and age, meant Grant was in control of them. These slaves were not freed until 1865 when Missouri officially abolished slavery.
Which were left to his daughter in his will. Yet you claim Lee owned none.
However the family owned four others under his wife's name, although Grant himself was responsible for supervising them.
For the sake of this argument let's assume that is true. What you are saying is that Lee's wife inherits close to 200 slaves but Lee doesn't own them. Grant's wife is given several slaves by her father and all of a sudden they belong to Grant?
In reality, Grant's wife had the use of four slaves. Title to the slaves do not appear to have been passed on to Julia Grant, much less her husband. The Dent family slaves were, according to Julia Dent Grant's autobiography, freed shortly after the Emancipation Proclamation. There is reason to believe that this was a bit of an exaggeration on Julia Grant's part. Evidence indicates that the Dent family slaves just ran off over time after the Emancipation Proclamation, and that includes the slaves that Julia Grant had use of. In any case there are no accounts of any slaves accompanying Mrs. Grant on her visits to her husband or on any of her other travels after early 1863.