There’s a machlokes if goyim can have a kinyan in other goyim as avadim; the government would have a “kim li” on those shitos, to retain their status as muchzak. You can’t be motzi mamon from them to the descendants of slave owners. Also, the slave owners didn’t expect reasonably to be reimbursed, so they were mochel even on the tzad that they’re entitled to it.
As was pointed out, melech poretz geder applies here – the government can take what it wants, when it wants, and doesn’t even have to reimburse you (according to manu shitos)