Relations can be had with slaves through the following means:
- A free male may have relations with female slaves that he owns himself.
- A free male may marry a female slave owned by someone else.
- A male slave may marry a female slave.
- A male slave may marry a free woman.
I am assuming that you are asking about the second case: a free man marrying a slave woman.
The following is a summary of the rules mentioned in الموسوعة الفقهية Wherever a difference among the schools has been recorded I have put a خ to indicate it.
This marriage is forbidden in principle but exists as a concession when certain conditions exist. It is better to not do this.
وأن تصبروا خير لكم
but to be patient is better for you
— Quran 4:25
The marriage is conditional to the following requirements regarding the man who wants to contract such a marriage:
He should not be married to a free wife with whom he can have relations and he should not be able to marry a free woman (e.g. because of not being able to afford mahr) as the Quran says:
ومن لم يستطع منكم طولا أن ينكح المحصنات المؤمنات فمن ما ملكت أيمانكم من فتياتكم المؤمنات
And whoever among you cannot [find] the means to marry free, believing women, then [he may marry] from those whom your right hands possess of believing slave girls.
— Quran 4:25
He should not own a female slave with whom he can have relations and He should not have the ability to buy a slave with whom he can have relations. This is because a slave can satisfy his desires and a slave is a fair deal of wealth which can be used as mahr.
He should have the fear that if he does not marry the slave, then he would fall into Zina خ , as the Quran says:
ذلك لمن خشي العنت منكم
This [allowance] is for him among you who fears sin
— Quran 4:25
If any of the above conditions no longer hold true some time after his marriage, then the marriage will remain intact and not be annulled.خ
The slave should be a Muslim and not a Kitabee خ as the Quran says:
من فتياتكم المؤمنات
of believing slave girls.
— Quran 4:25
It is not permitted for a free person to marry their own slave or the slave of their children. Though they can free them first and then marry them.
If a person marries a slave and then receives ownership over her (e.g. through sale, gift, inheritance) the marriage is annulled.
The marriage can only be done by the permission of the owner of the slave as the Quran says:
فانكحوهن بإذن أهلهن
So marry them with the permission of their people
— Quran 4:25
For the contract of marriage, the Wali of the slave will be her owner, unless the owner is a femaleخ in which case the Wali will be the male Wali of the owner who will arrange the marriage with the permission of the female owner.
The mahr (dower) will go to the owner of the slave.خ
The owner can no longer have sexual relations with his slave who has been married off, she will observe awrah with regard to him as done with a mahram.
The nafaqah (maintenance) of the slave is the responsibility of the husband when she is with him and the responsibility of the owner when she is serving himخ.
Children born of such a marriage will be slaves owned by the owner of the wife, unless the husband has made it a condition in the contract that they will be freed. This is opposed to the case of master-slave relations where the slave becomes an Umme Walad and the children are free children of the master in every respect.
If the children are freed by the owner, their nafaqah is the responsibility of the father, unless he dies or is poor in which case it is the responsibility of the owner.
Azl (coitus interruptus) may not be done with a slave wife without the permission of the slave and the permission of the owner خ of the slave.
A slave who has been married has the right division of nights from her husband, unlike in slave-master relations.
References:
الموسوعة الفقهية الكويتية Vol 23 Page 49 - 55
Tafsir al-Qurtubi on 4:25