Jefferson's practice of slavery was entirely evil and inexcusable.
Also understandable, given the context of the time in which he lived.
You might be interested in his one "freeing" of a slave, the man who was freed died soon after of alcoholism and depression. Jefferson was highly moved by this, and from what I've read he consider it his own fault the man died since he was freed without the ability to take care of himself.
I don't believe that the concept of "freeing" the slaves and hiring them back on as contract laborers ever occurred to him, even though such an action seems reasonable with 20/20 hindsight.
What bearing does this have on the quote? Are we to judge ideas on the virtue of the man who articulated them, or on their own merits?
Pay me 50 bitcoins or I will reveal to the world that the principles you say you hold dear were voiced by rich, white, slave owners.