This was Lincoln's original plan, but since he died, it never happened.
Lincoln never proposed colonization of freed blacks in Africa. By 1860, the Liberia project was moribund.
Lincoln did very tentatively suggest colonization in Nicaragua, but as there was no support whatever among blacks for it, he dropped it at once. This was in 1863, IIRC.
By the end of the War, he was openly hinting at eventual citizenship for blacks; that sent Booth into a rage. He killed Lincoln a few days later.
To have the US embark on a major "colonization" program after the War, one would have to have a different President, not a Republican. Note that even Andrew Johnson, a white supremacist Southerner, didn't push for colonization.
Hmm. Suppose Atlanta holds out till just after the election. Then it falls; and a delayed "Crater" attack succeeds about the same time. Richmond falls; Union cavalry catch Davis and the Confederate cabinet fleeing Richmond. Whoops! The Democrats "peace platform" is blown to bits.
Lincoln pushes the 13th Amendment as OTL, but with a Democrat majority incoming, it requires a major concession to get it through Congress: a clause restricting citizenship to "persons of the white race".
The incoming President (not McClellan) is someone who viscerally dislikes blacks; before the War he was a Free-Soiler to preserve the Territories for white men only. He wants them out of the country. His followers in many parts of the country enact draconian Black Codes and laws excluding blacks from entire states. (Such laws existed before the War, but were never rigidly enforced.)
On the other side, faced with perpetual inferiority, black leaders return to colonization, supported by many former Abolitionists. The President endorses this; his working-class followers welcome the removal of black competition. "Transportation" to Africa becomes a common sentence for black "criminals" (real or nominal).
So over the next 25 years, a million US blacks migrate or are transported to Liberia, which becomes an official US protectorate.