No. Lincoln's commitment to the preservation of the Union is entirely in keeping with his whole political career and worldview up to the day of his death. No historian I'm aware of has ever suggested such a theory, nor mustered evidence for it. Lincoln scholarship is vast and goes way out into the weeds, but I don't know that it's ever gone there. Nor is there any reason to think it should, given no one seems to have claimed to bribe him, he doesn't need a bribe to do what he was already inclined to do for his whole public life, and I've never even seen anybody at the time argue that he was bribed for it. Instead his enemies waging a treasonous war to save slavery charged him with "fanaticism" for suggesting that maybe slavery ought to be slowly ended over the course of a century or so.