It may be getting confused with the Peace Conference in Washington in February, where the idea of such a clause to a constitutional amendment to appease the southern states was discussed.
POTCS Davis sent SOS Toombs and a few other CSA officials to DC to negotiate with the USA in March. IMO The whole war could have been avoided if Lincoln recognized the CSA (7 states at that time) with reunification in the future.
That is the problem, my studies of this are over 40 years old, but I do remember that the return of slaves was a significant point in several different discussions, and the abolitionist groups would never have agreed to anything like that. I also do not think that Lincoln would have agreed to that.