quran mentions believing slave so muslim can be taken as slaves
by other muslims?
Wed not idolatresses till they believe; for lo! a believing bondwoman is better than an idolatress though she please you; and give not your daughters in marriage to idolaters till they believe, for lo! a believing slave is better than an idolater though he please you. These invite unto the Fire, and Allah inviteth unto the Garden, and unto forgiveness by His grace, and expoundeth His revelations to mankind that haply they may remember