As for Constitutionality, I think a 5th Amendment challenge would be in order.
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.
And from the Fourteenth Amendment: Section 1:
No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
And Section 4:
But neither the United States nor any State shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.
Which would be a roundabout argument, but thus: because the value of slaves forcibly emancipated or otherwise manumitted was forbidden by law to be paid to the former owners, that value becomes a gift to those freed, even if done so by government itself. The former owners suffered the loss of their investments in those slaves (property, as defined), without compensation for that loss.
Reparations were already done at that time, by force of law, and without compensation to the former owners, as the bond value of each freed slave was not recovered by any party, and thus devolved to the former slave, who previously would have had to remit that amount to gain freedom from servitude.