Is freeing slaves considered an expiation for accidentally killing someone, and is it permissible to enslave people?

The expiation for unintentional killing is the freeing of a believing slave, if found. The validity of the manumission depends on the validity of the seller's ownership of the slave. If the ownership is valid, then selling and freeing the slave is permissible. However, if the slave was enslaved unjustly, then neither selling them nor using them for expiation is valid. Please consult scholars and historians in the questioner's country to ascertain the validity of ownership.