God-given, unalienable, individual rights precede and supersede all man made laws and constitutions. This has been a bedrock principle of western civilization going back through Blackstone, and Locke, and Aquinas, all the way to Cicero. Surely you know that.
If anything is "ex post facto," in the truest sense, it is the illegitimate claim to a "right" to commit the gross wrong of man-stealing and then robbing generations of those who have been stolen of their lives, their liberty, and their marriage, family, and property rights.
Yeah, give that a rest for a moment. Ex Post Facto is referring to "After the Fact", as in making something a law, (Or suddenly morally outrageous) "after the fact."
Before the war? Slavery was A-OK with the Union. After the war? Slavery was an abomination and justified the deaths of 600,000 men and the devastation of vast swaths of populated areas.
Pushing this argument actually makes good financial sense. By declaring it an abomination, they didn't have to pay anyone for what they took away. Billions of dollars in financial assets just evaporated as punishment because the Southern states fought back.
The FedZilla didn't have to pay any claims, because they retroactively made a previously legal thing into an illegal thing. Now that worked out just swell for them, didn't it?