You may buy slaves
Leviticus 25:44

• (Leviticus 25:44) - "‘As for your male and female slaves whom you may have—you may acquire male and female slaves from the pagan nations that are around you."  

     In the fallen world that mankind had created, slavery was a reality.  God permitted its existence and worked within its system.  Slaves were more domestic servants than oppressed field workers.  Slaves could be the captives of war (Num. 31:25-47), subjects of debt to be worked off (2 Kings 4:1), born into slavery (Gen. 17:12-13), or entered into voluntarily (Exodus 21:5-6).  In the Ancient Near East, some slaves were able to own other slaves and even conduct business and in Exodus 25:2 a slave was required to be set free after six years of service.  Though slavery carries a very negative connotation here in America, it was not nearly as bad it was here in the first 100 years of our nation's existence.  
     As I said above, God works within the fallen system to bring about His will.  Slavery was permitted by God and rules of treatment of slaves were given so that they would not be mistreated.