Hukum related to Re-Enslavement of Freed Kafir Slave

السلام عليكم Is there any specific ahkam related to the re-enslavement of a kafir slave who was freed by his Muslim master but traveled and settled in the land of Dar al Harb & muslim seized him in Jihad or was captured by other Kuffar? Is it permissible for the Muslims to buy and keep him/her as a slave or he must be set free? Note: Slave is still Kafir