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

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.