I'm not so certain that it is that simple, Edward. Remember, we are not dealing with a "law" we are dealing with the actual Constitution itself. The "ex post facto" clause in the Constitution is just that, a clause. We are talking here about an amendment, not a law. Changing the Constitution changes the rules completely. As to the Legal and constitutional specifics of slavery, the 13th Amendment and compensation: I think it would be safe to say that with an unchallenged unionist congress, president and a completely prostrate South, the law as well as application of the Constitution was pretty much be made up as they went based on what the North wanted. Remember that Southern states could not be readmitted with representation unless they ratified the 13th Amendment. But until they were readmitted with representation, how could they ratify an amendment? Pretty crazy stuff.