Intercourse with a slave woman is not regarded as zina (adultery)

Firstly: 

This hadeeth was narrated by al-Bukhaari
(2542) from Ibn Muhayreez who said: I saw Abu Sa’eed (may Allaah be pleased with him) and I asked him. He said: We went out with the Messenger of
Allaah (peace and blessings of Allaah be upon him) on the campaign of Banu al-Mustaliq, and we captured some prisoners from among the Arabs.
We desired women and the period of abstention was hard for us, and we wanted to engage in ‘azl (coitus interruptus). We asked the Messenger of
Allaah (peace and blessings of Allaah be upon him) and he said, “There is no point in doing that, for there is no soul which Allaah has
decreed should exist until the Day of Resurrection but it will come into existence.”
According to another report, They captured some female prisoners and wanted to be intimate with them without them becoming pregnant. They asked
the Prophet (peace and blessings of Allaah be upon him) about ‘azl and he said, “There is no point in doing that, for Allaah has decreed who
should be created until the Day of Resurrection.” 

This hadeeth was also narrated by Muslim
(1438), whose version says: We captured some women of the Arabs and we had been abstinent for a long time; and we wanted to be able to sell them,
but we wanted to engage in intimacy with coitus interruptus. We said, “Shall we do that when the Messenger of Allaah (peace and blessings of
Allaah be upon him) is among us without asking him about it?” So we asked the Messenger of Allaah (peace and blessings of Allaah be upon him)
and he said, “There is no point in doing that, for Allaah has not decreed that any soul should be created until the Day of Resurrection but it
will come into existence.” 

It may be understood from the hadeeth that
those who wanted to engage in ‘azl did so for two reasons: they did not want the women to become pregnant, and they wanted to be able to sell them
– if a slave woman got pregnant she could not be sold. 

It may also be understood that ‘azl (coitus
interruptus) does not change anything. If Allaah decrees that a child should be born, water (semen) will come out before the man realizes it. 

Secondly: 

Allaah has permitted intimacy with a slave
woman if the man owns her. This is not regarded as adultery as suggested in the question. Allaah says, describing the believers (interpretation of
the meaning): 

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

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

[al-Mu’minoon 23:5]

What is meant by “those whom their right
hands possess” is slave women or concubines.  See also question no. 10382,
12562. 

Once this is understood, it should be noted
that what is suggested in the question, that this was zina, never occurred to the Sahaabah (may Allaah be pleased with them). What they were
asking about was the ruling on practicing ‘azl with the slave women whom they had acquired in the course of jihad. 

Moreover ‘azl may be done with a concubine or
with a wife, if she agrees to that. See question no. 11885. 

And Allaah knows best.