Upvote:7
This is probably referring to the preliminary announcement of the Emancipation Proclamation. The preliminary version was announced on 22 Sept 1862, announcing the Emancipation Proclamation would take effect on 1 Jan 1863, 100 days later. This is outlined at a University of Michigan site:
On September 22, 1862, Lincoln initiated a debate when he issued what is often termed the preliminary Emancipation Proclamation. Contemplating how slaveryβs abolition would undermine the Confederacy, the President aimed to use the threat of emancipation to force Southern states to rejoin the Union. Lincoln proposed that 100 days hence, on January 1, 1863, slavery would be abolished in all rebellious territories, with two conditions: efforts to colonize or remove former slaves to a separate territory would continue, and loyal slaveholders would be compensated for the loss of their property.
Note the portion about loyal slaveholders would be compensated. This indicated it was not an offer for rebellious states to keep their slaves, so that aspect of the query is incorrect.