The assertion that the new nation could have "easily" financed the purchase of the slaves is sufficient to discredit this book. That comment shows he either knows little about the era or that he is lying. I suspect the latter.
It wouldn't have been any different-in principle-than the scheme-which was eventually adopted by Abraham Lincoln-of compensated manumission in the (slave) border states that remained in the Union, after the outset of the Civil War.