Is Al-Amma (a mistress) allowed in Islam? If it is, then according to what conditions and what are the rule of having them?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The answer to this question can be summed up as follows:

Islam came at a time when slavery was widespread among all nations of the earth. According to the people of that period, it made no difference if the slave was captured in a lawful war, aggressive hostility, or by way of trickery or betrayal. When Islam came, it narrowed and restricted this system. It strongly condemned the selling and enslavement of free men and restricted slavery to the captives captured by way of lawful Jihaad. It then made efforts to liberate the slaves and vividly encouraged their freedom by opening and widening ways that led to slave manumission, like expiation for a false testimony, Thihaar [claiming one's wife to become sexually impermissible for him (like an unmarriageable woman) by saying words such as, "You are to me like the back of my mother (i.e. it is unlawful for me to have sexual intercourse with you just as it is unlawful with my mother)] and murder. This is in addition to encouraging and emphasizing being kind to the bondmen (or slaves), teaching them, honoring them and assisting them.

However, whoever owns a bondsman in a Sharee’ah approved way, it is lawful for him to sell him or present him as a gift. It is also lawful for the master of a bondwoman to have sexual relationship with her as long as she is not married and not forbidden to him in marriage due to lineage or any other reasons. Allah Says (what means): {And those who guard their chastity (i.e. private parts, from illegal sexual acts) Except from their wives or (the captives and slaves) that their right hands possess, for then, they are free from blame.} [Quran 23: 5-6]

There are Islamic legal rulings that relate to having sexual relations with a bondwoman. Some of these rules are: If she bears him a child, she becomes the mother of his child, and therefore, should not be sold; she should be freed after the death of her master and so on.

The master of a bondwoman can marry her off to a bondsman or a free man if the conditions that make this lawful are complete.

If Muslims fight the disbelievers, and take their women as captives, they can grant favor to them by releasing them, or accepting ransom from them, or taking them as bondwomen. If a bondwoman who is married to a disbeliever falls in the share of one of the Muslims, it is lawful for him to have marital intercourse with her if it is proved that she is not pregnant and after she has completed her menstruation. The evidence for this is the Hadeeth reported by Muslim on the authority of Abu Sa’eed Al-Khudri that the Messenger of Allah sent a troop to Awtaas. This troop encountered the enemies, fought them, and defeated them. They also took some female captives. Some companions of the Prophet felt uncomfortable having sexual relationship with these bondwomen because of their husbands who were polytheists. Allah then revealed, (what means): {Also (forbidden to you for marriage are) women already married, except those (slaves) whom your right hands possesses.} [Quran 4: 24] These women are therefore lawful for you as long as they have completed their waiting period.

Abu Dawood also reported that the Messenger of Allah said, “No pregnant bondwoman should be subject of intercourse until she delivers her baby, and no non-pregnant woman should be subject of intercourse until after she has completed one menstruation.”

For more benefit on slavery and having sex with slave women, please refer to Fataawa 357964, 256111, 386737, 386479, 84431, 83687, and 82182.

Allah knows best.