How would reparations necessarily be unlawful? If it's a law, that would make the act lawful, wouldn't it? Just like slavery itself before 186t?

no

Extortion is the unlawful act of obtaining money, property, or services from another person through the use of force, threats, or coercion. It involves an illegitimate demand or request made by one party to another, often accompanied by a credible threat of harm, injury, or damage to the victim’s person, property, or reputation.

Slavery was lawful before 1865.

