In the article you linked I did not see any reference to where such order was voted on and approved by congress nor where the president of the nation signed such an order after the approval of congress. The article say's that Lincoln approved it but that is one version of the story.
Another version is below,
"The orders were issued following
Sherman's March to the Sea. They were intended to address the immediate problem of dealing with the tens of thousands of
African refugees who had joined Sherman's march in search of protection and sustenance, and "to assure the harmony of action in the area of operations".
[3] His intention was for the order to be a temporary measure to address an immediate problem, and not to grant permanent ownership of the land to the freedmen, although most of the recipients assumed otherwise.
[4] General Sherman issued his orders four days after
meeting in
Savannah, Georgia with twenty local African ministers and lay leaders and with
U.S. Secretary of War Edwin M. Stanton. Brig. Gen.
Rufus Saxton, an
abolitionist from
Massachusetts who had previously organized the recruitment of
African soldiers for the
Union Army, was put in charge of implementing the orders.
[5]
The orders had little concrete effect, and President
Andrew Johnson issued a proclamation that returned the lands to southern owners that took a loyalty oath"
Take note of the "His intention was for the order to be a temporary measure to address an immediate problem, and not to grant permanent ownership of the land to the freedmen,".
I do not see proof that this order was valid and legal to begin with and taken through the proper channels of government to be a legal binding order from the outset.