Below is an example. Obviously, General Lee did not advocate this action and the pretense of "continued as the Army..." is not supported as a policy in this brief narrative. My question is about whether this was a common practice during the campaign or some isolated incident that Yankees could use to claim moral superiority. The last line is the heart of the matter. How does one prove a lie?

from Stephen Spears, "Gettysburg", (Houghton Mifflin, 2003), page 111-112:

"The pursuit and capture of blacks, initiated by Jenkins's cavalry in Chambersburg, continued as the Army of Northern Virginia crossed the Pennsylvania line. In Mercerburg , for example, professor Phillip Schaff recorded in in his diary that ' public and private houses were ransacked, horses, cows, sheep, and provisions stolen day by day without mercy, negroes captured and carried back into slavery (even such as I know to have been born and raised on free soil) and many other outrages committed'." "Slave-catching was practiced by the infantry as well. William S. Christian, colonel of the 55th Virginia...wrote his wife...on June 28, 'we took a lot of negroes yesterday. I was offered my choice but as I could not get them back home I would not take them....In fact, my humanity revolted at taking the poor devils away from their homes'. " "...Slave-catching...was without question widely and officially tolerated.... Of various ugly incidents stemming from Lee's Pennsylvania invasion, this was surely the ugliest".