Maybe yes, maybe no.
So much of what Obama has said about his past has turned out not to be true. Why should the story of his book deal be the truth when so much of his past isn’t true.
If it was just Obama’s word about any of this, I’d be the last person alive to believe him. However, all the details I mentioned, including the size of the advance and the fact that the agent approached Obama out of the blue, are matters of public record.