Did Northern troops attempt to re-enslave African Americans in Southern plantations during the Civil War?
Short Answer #1:
Not Just Northern Troops. It was the North's policy for the first year of the war, to be followed by all of it's generals. Generals who did not follow this policy (like John Fremont) and freed slaves risked losing their ...
From McPherson's Battle Cry of Freedom, p.711:
The need of northern and British textile mills for cotton also caused
the army to put many freed people to work growing cotton—often on the
same plantations where they had done the same work as slaves. Some of
these plantations remained in government hands and were administered
by "labor superintendents&...
This is referring to the area around New Orleans, ca. 1863-5. The Emancipation Proclamation didn't apply to lower Louisiana, which was already under Union occupation. It only applied to areas that were in a state of insurrection on January 1, 1863. Slaves in lower Louisiana remained enslaved until the passage of the 13th amendment on December 6, 1865.