Ibn Majah

Benefits and Issues:
➊
"Umm Walad" refers to a slave woman of a person from whom the owner has a child, whether it is only one son or daughter.
The plural of Umm Walad is "Ummahat Awlad."

➋
When a master establishes sexual relations with his slave woman, the child born as a result is free.

➌
Hafiz Ibn Hajar rahimahullah said:
Imam Bukhari rahimahullah has mentioned two hadiths in this chapter (Kitab al-'Itq, Chapter of Umm Walad).
There is no evidence for any legal ruling in them.
In my view, there was a strong difference of opinion among the Salaf (the Companions radi Allahu anhum) on this issue, although among the later scholars there was consensus on its prohibition, to the extent that even Imam Ibn Hazm and his followers, the Ahl al-Zahir, became proponents of the prohibition of selling Umm Walad. Therefore, the second opinion can only be considered rare. (Fath al-Bari, the aforementioned chapter).

Benefits and Issues:
Is it permissible to sell an umm walad (a slave woman who has borne her master a child) or not? There are both opinions among the scholars on this matter.
Some hold it to be permissible, while others hold it to be impermissible.
Imam Shawkani's opinion is that, as a precaution, the view of impermissibility is more correct.
For details, see: (Nayl al-Awtar, Book of Emancipation, Chapter: What Has Been Reported Regarding Umm Walad) and Allah knows best.

Takhrij:
«أخرجه ابن ماجه، العتق، باب أمهات الأولاد، حديث:2517، والنسائي في الكبرٰي:3 /199، حديث:5039، والدارقطني:4 /135، وابن حبان(الإحسان):6 /265، حديث:4308.»©Explanation:
This hadith implies the permissibility of selling an umm walad (a slave woman who has borne a child to her master), but most scholars hold its prohibition. In fact, a group among the later scholars has even claimed consensus (ijma‘) on this, and has stated that the prohibition by Umar radi Allahu anhu of selling her, and the fact that the noble Companions radi Allahu anhum did not oppose him, is evidence that they were in agreement regarding its prohibition. Their position is further supported by the statement of the Noble Prophet sallallahu alayhi wa sallam: «أَیُّمَا امْرَأَۃٍ وَلَدَتْ مِنْ سَیِّدِھَا فَھِيَ مُعْتَقَۃٌ عَنْ دُبُرٍ مِنْہُ» “Any slave woman who gives birth to a child by her master becomes free after his death.”
(Musnad Ahmad: 1/303, 317, 320; Sunan Ibn Majah, Book of Emancipation, Chapter: Ummuhat al-Awlad, Hadith: 2515) There are many other chains and wordings of this hadith as well.
See: (Awn al-Ma‘bud: 4/47) The reason for deducing from this hadith is that it indicates a general principle and comprehensive rule, namely that a slave woman who is an umm walad becomes free after the death of her master.
Acting upon this principle is only possible if the sale of an umm walad slave woman is prohibited.
As for the narration of Jabir radi Allahu anhu, it carries many possibilities.
At times, mere silence regarding an action does not provide such a definitive specification as would be able to counter a general principle and comprehensive rule.