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.
Sheikh Muhammed Salih Al-Munajjid