Lincoln could have bought every slave in the South and resettled them in the North for a fraction of the human and economic cost of the Civil War.
Obviously, the North was never going to go along with that idea.
Free the slaves? Oh yeah, man!
Live next door to a former slave family, compete for low skill jobs with former slaves, and compete politically against a 4 million person voting bloc?
Um, no thank you - in both the North and the South.
Lincoln knew that and signed the Compensated Emancipation Act in 1862 which eliminated slaver in the District of Columbia. He also advocated a version of it for the slave states that remained in the Union but it was narrowly defeated.
Slave states had no interest in changing.