What was the alternative?
Lincoln could have negotiated with the Southern States to return to the Union and purchase every slave in the south for much less than human and economic cost of the American Civil War.
What it the South didn't want to sell their slaves? Which they didn't. And if they did, what would prevent them from buying more?
The idea of Lincoln buying the slaves instead of going on with the Civil War comes from the book “The Peculiar Institution” by Kenneth M. Stamp. Mr. Stamp also gives us the economic costs in dollars and shows us that that it would have been financially less expensive. Some slaves were able to buy their freedom. Lincoln was part of an organization that was sending freed slaves to Liberia which was a U.S. colony at the time.