Only if the slave owners are fairly compensated for their human chattel. Otherwise it would be government theft, and arguably Unconstitutional, as as uncompensated government taking. The analogy is with zoning. You can downzone and reduce the value of a property, but can't zone down to where the property has no value at all, because next to nothing can be done with it with any economic value. And freeing the slaves would be equivalent to such a Draconian downzoning. But bearing this analogy in mind, perhaps it would be Constitutional to free slaves who reach a certain age, say 55 or something.
But the downside of slave freedom at age 55, is that at that point, it might be hard for them at such an advanced age, where their ability to work in the fields is so limited, to earn a gainful living, and they would either starve to death, or be a tax burden on the state. And the slave owners might be incentivized to mistreat them more say, when they reach the age of 50, to extract the max economic value from them as they can, over the short term, rather than focusing on the longer term economic life span of the slave, which would incentivize better slave treatment, i.e, to better maintain the health of their chattel.
Darn this is complicated. Let me think about this some more. I'll get back to you.
Doesn't this thread belong on the Individual Politics board?