And the classic all-time, Turtledove's Guns of the South. Robert E. Lee runs for the presidency of the Confederacy, and one of the planks of his platform is a plan where abolitionists can put their money where their mouth is and purchase the freedom of any slave at a price to be decided by qualified appraiser, and the slave's "owner" cannot refuse this price.
Don't want to give away the whole story, but there's an everyman character who directly defends this as a good and useful safety valve.
* I think there's a surprising percentage of Alternate History members who have not read this novel, and that's perfectly okay. Harry Turtledove is not everyone's cup of tea.