It is not permissible for a master to have intercourse with his married slave woman
Question

                Is a man permitted to have sexual intercourse with a slave woman who is married?
Praise be to Allah. 
          It is not permissible for a man to have intercourse with his slave woman 
          who is married; whoever does that has committed a haraam (forbidden) 
          action and is to be punished. 
Ibn Qudaamah 
          said in al-Mughni: if he marries off his slave woman, then it 
          is forbidden for him to have intercourse with her… There is no doubt 
          and no dispute concerning the prohibition on his having intercourse 
          with her, for she is now permissible to her husband, and no woman can 
          be permissible to two men. If he has intercourse with her then he is 
          guilty of sin and must be punished. Ahmad said, he should be flogged 
          but not stoned, i.e., his punishment (ta’zeer) should take the form 
          of flogging. 
Vol. 9, 
          p. 497.