As President, Abraham Lincoln had the power to issue the Emancipation Proclamation. But it was an executive order, not a legislative mandate. The President declared the slaves free, but he had no power to regulate how former "masters" would treat former "possessions."
Presidents execute laws - they don't make them. And, significantly, Lincoln's Proclamation only freed slaves in the Confederate states. (Slaves in states which remained in the Union were not freed until the 13th Amendment was ratified, in December of 1865.)
What force of law would the Proclamation have in states that were no longer part of the United States? Not much, if you were a slave-owner living in a Confederate state. Not much, if your President was Jefferson Davis.
Lincoln said former slaves could fight in the Civil War, and be fully paid for it - and they did. After the President signed the Proclamation, Union recruiters actively sought African-American men to serve as soldiers. According to the Library of Congress, by war's end one out of every eight Northern soldiers was a black man.
But ... who would make sure those soldiers, and their families, were safe once they returned home from battle?
And ... since Congress never passed any laws to create a process by which former slaves would become fully-participating citizens, there was room - lots of room - for future problems.