Lincoln knew that if four million slaves were released, they could then be able to help the progress in the North.
Booth wanted to kill Lincoln when Lincoln advocated going one step further after the war and proposed giving the slaves the right to vote.
Lincoln's proprosals progressed as his own views changed.
I see no evidence in the records of freed blacks heading north to "help" in any way in the decades after the Civil War. The North had an abudance or labor available due to immigration from Europe. It was not until WWI when immigration all but shut down when sizable numbers of blacks moved north for factory jobs, a trend that grew even greater during WWII.