Douglas Southall Freeman talks about Lee's slave ownership earlier in his life. He inherited some from his mother on her death in Lee's own letters talk about owning slaves going back to the 1840sThe Library of Congress has documentation of two Lee slaves, William and Rosabella Burke, who Lee freed in the early 1850s and whose passage to Liberia was paid by Lee. Burke later graduated from a Presbyterian seminary in Monrovia and was an minister. Mrs. Burke corresponded with Mrs. Lee for years prior to the rebellion. Here's a link to the site: Source
Interesting reference to the Lee's but I didn't see anything suggesting that Robert E. Lee ever purposefully acquired slaves of his own.
Not sure that it matters. Whatever Lee's association with slavery it did not disqualify him in the eyes of the Great Emancipator who unsuccessfully recruited Lee to attack people in the South.
When that did not work for Lincoln he turned to others who had associations with slavery to do the work. Grant comes to mind.