What is the ruling on intimacy with slave women?
Question

                In Ar-Raheeo Al Makhtum (The Sealed Nectar) the author says in the section called  "The Prophet Household" that the Prophet (S.A.W.) had four concubines

1. Why is it that having concubines is not haram? 

2. Can other muslims have concubines?.
Praise be to Allah.
    Allaah is Most Wise in the laws that He decrees for His 
    slaves, but this wisdom is only apparent to those who seek it and believe 
    that Allaah is All Wise, and looks at the interests that are served by the 
    laws of Allaah which no man can see unless he ponders the matter, especially 
    when there are those who attack those laws because they go against what they 
    think are right and wise. 

    With regard to your question about it being permissible for a 
    master to be intimate with his slave woman, the answer is that that is 
    because Allaah has permitted it. Allaah says (interpretation of the 
    meaning): 

    “And those who guard their chastity (i.e. private parts, 
    from illegal sexual acts)

    6. Except from their wives or (the slaves) that their 
    right hands possess, for then, they are free from blame”

    [al-Mu’minoon 23:6; al-Ma’aarij 70:30]
That is subject to the 
    condition that he has acquired her in a proper manner, and that this slave 
    woman has not been given by her master in marriage to another man to whom 
    she is still married. The reason why this is permitted is that this slave 
    woman belongs to him, either because he has paid money to buy her or he has 
    fought for the sake of Allaah (and acquired her among the war booty). 
Shaykh al-Shanqeeti (may 
    Allaah have mercy on him) said: The reason for which people may be taken as 
    slaves is if they are kaafirs who are waging war against Allaah and His 
    Messenger. If Allaah grants victory to the mujaahid Muslims, who are 
    offering their souls, their wealth and all their resources and everything 
    that Allaah has given them so that the word of Allaah might prevail over the 
    kuffaar, then these kuffaar may become slaves, unless the imam chooses to 
    let them go or to ransom them if that serves the interests of the Muslims. 

    Adwa’ al-Bayaan, 3/387 

    Shaykh al-Shanqeeti (may Allaah have mercy on him) said: 

    If it is said: If the slave is a Muslim, how can he be kept 
    as a slave if the reason for which people may be taken as slaves is if they 
    are kaafirs who are waging war against Allaah and His Messenger? The answer 
    is that the basic principle which is well known to the scholars and all wise 
    people is that a right that is already established by shar’i means cannot be 
    superceded by rights that are established later on. 

    When the Muslims take prisoners of war, they are given the 
    right to enslave them by the law of the Creator of all, and He is the 
    All-Wise, All-Aware. If this right is established, then the slave becomes a 
    Muslim after that, his right to be freed from slavery because of his Islam 
    is superceded by the right of the mujaahid whose right to enslave him took 
    effect before he was a Muslim. It is not just or fair to waive the former 
    right because of a latter right, as is well known to all wise people. 

    Yes, it is good for the owner to set him free if he becomes 
    Muslim, and Islam enjoins that and encourages it, and opens the door to 
    doing so in many ways – he is referring to the fact that Allaah has decreed 
    that when expiation takes the form of freeing a slave, the slave in question 
    should be a Muslim –  

    Glory be to the All Wise, All Aware: “And the Word of your 
    Lord has been fulfilled in truth and in justice. None can change His Words. 
    And He is the All‑Hearer, the All‑Knower” [al-An’aam 6:115]. 
“In truth” means in what 
    He has told us; “in justice” means in His rulings. 

    Undoubtedly that justice includes owning slaves and other 
    rulings that are mentioned in the Qur’aan. 

    Adwa’ al-Bayaan, 3/389. 
With regard to Muslims 
    owning slaves, he should be very careful to establish that those who are 
    bought or sold now are indeed slaves, because Islam has limited the sources 
    of slaves which were many before the coming of Muhammad  (peace and 
    blessings of Allaah be upon him), and has allowed only one source, which is 
    kaafir prisoners of war, when the kuffaar are fighting the Muslims. There is 
    no other way in which they may be enslaved except those who are captured as 
    a result of fighting between kaffirs and Muslims, or their children.   See 
    also questions no. 
    26067 and 12562 
    for more information. 

    And Allaah knows best.