If you check out the latest LRC podcast, Lew and historian John V. Denson talk about Lincoln (starts at 44:20). Denson mentions that during the Hampton Roads Peace Conference, Lincoln did not make the South accept the abolition of slavery as a condition for peace. In fact, he said that if the South rejoined the Union, the South would have enough votes to strike down the 13th amendment (this point is made at about 48:00).
Here’s an article from Denson about this conference. Denson summarizes the conversation at the conference, which seems to clearly show that Lincoln wanted the re-unite the Union, slavery or no slavery.