The report has been classed as authentic by al-Albani in Irwa' al-Ghaleel and also by some other scholars. It is an athar about Ibn Umar. Not a 'hadith' (i.e. not something said or done by the Prophet ﷺ nor something done during his lifetime such that his approval can be assumed), and not about Umar (rather it is about his son). The content of the report is that Ibn Umar would look and touch a slave woman when he intended to buy her. This report can be and has been used as a source of law as explained in the appendix below.
It is permissible to look at or touch a slave woman when buying her. This is customary and necessary as it is done to detect any physical defects or to verify qualities which are used to appraise the price which is going to be paid. Just like one can examine an expensive house, vehicle, furniture, dress or even fruits and vegetables which they wish to buy, they can also examine the slaves whom they intend to buy.
It is considered permissible because of this necessity, similar to how it is permitted to look when proposing marriage, or when witnessing Zina, or when performing medical treatment etc. As for the extent of what can be seen or touched it is disagreed upon as covered under your question on the 'awrah of a slave woman. And in this report it is stated that Ibn Umar touched the slave from behind a cloth barrier.
قال: "ولا بأس بأن يمس ذلك إذا أراد الشراء، وإن خاف أن يشتهي" كذا ذكره في المختصر، وأطلق أيضا في الجامع الصغير ولم يفصل. قال مشايخنا رحمهم الله: يباح النظر في هذه الحالة وإن اشتهى للضرورة، ولا يباح المس إذا اشتهى أو كان أكبر رأيه ذلك؛ لأنه نوع استمتاع، وفي غير حالة الشراء يباح النظر والمس بشرط عدم الشهوة.
— Hidayah sharḥ Bidayat al-Mubtadi
This is to elaborate more on why scholars adopt this ruling based on the saying of Ibn Umar and why the prohibition of looking or touching a free woman does not apply to slaves.
Looking at a free woman when proposing marriage is permitted, even without her permission:
قال رسول الله صلى الله عليه وسلم " إذا خطب أحدكم المرأة فإن استطاع أن ينظر إلى ما يدعوه إلى نكاحها فليفعل " . قال فخطبت جارية فكنت أتخبأ لها حتى رأيت منها ما دعاني إلى نكاحها وتزوجها فتزوجتها
The Prophet (ﷺ) said: When one of you asked a woman in marriage, if he is able to look at what will induce him to marry her, he should do so.
Jabir said: I asked a girl in marriage, I used to look at her secretly, until I looked at what induced me to marry her. I, therefore, married her.
— Abu Dawud
And rationally a slave woman who is being purchased is more deserving of being examined since more is at stake and because the rules of hijab for slaves are more relaxed:
فقال المسلمون إحدى أمهات المؤمنين أو مما ملكت يمينه، فقالوا إن حجبها فهى من أمهات المؤمنين، وإن لم يحجبها فهى مما ملكت يمينه
The Muslims wondered, "Is she (Saffiyya) considered as his wife or his slave girl?" Then they said, "If he orders her to veil herself, she will be one of the mothers of the Believers; but if he does not order her to veil herself, she will be a slave girl."
— Bukhari and Muslim
إذا زوج أحدكم خادمه عبده أو أجيره فلا ينظر إلى ما دون السرة وفوق الركبة
When one of you marries his female servant to his slave or to his employee, he should not look at her private part below the navel and above the knees.
— Abu Dawud
There is evidence from the Prophet ﷺ that touching slave women is permissible when there is no fear of fitnah as indicated in the ahadith:
كانت الأمة من إماء أهل المدينة لتأخذ بيد رسول الله صلى الله عليه وسلم فتنطلق به حيث شاءت
Any of the female slaves of Medina could take hold of the hand of Allah's Messenger (ﷺ) and take him wherever she wished.
وإنا حضرنا معه مرة طعاما فجاءت جارية كأنها تدفع فذهبت لتضع يدها في الطعام
Once we went with him to a dinner when a girl (the word means a minor or a slave) came rushing as if someone had been pursuing her. She was about to lay her hand on the food, when Allah's Messenger (ﷺ) caught her hand.
And purchase of a slave is a matter in which touching is even more deserving of being permissible because there is a necessity and because her hijab is more lenient.
Further, Ibn Umar was a sahabi who got his knowledge directly from the the Prophet ﷺ so it is possible that he was acting on what he learnt from the Prophet ﷺ. Otherwise it was his ijtihad and even his ijtihad is usable evidence as is known in the principles of jurisprudence. Especially when his saying is corroborated by what is attributed to other sahaba and tabieen.
Following a sahabi or a scholar does not mean that we are making them legislators instead of Allah. Rather we are accepting their interpretation of the Quran and Ahadih or following their ijtihad in a matter in which there is no clear ruling in the Quran and Sunnah.
يا أيها الذين آمنوا أطيعوا الله وأطيعوا الرسول وأولي الأمر منكم
O you who have believed, obey Allah and obey the Messenger and those in authority among you.
— Quran 4:59
A detractor who claims that touching or looking at a slave when purchasing is haram needs to provide a verse in the Quran or a hadith which clearly says that when talking about a "slave". There is no such verse nor hadith. Hence such a person is only acting on his whims if he is a layman and on his own interpretation otherwise - the understanding and ijtihad of a sahabi is more worthy of following because of their greater knowledge and piety.
Saying that a rule about the hijab of a (free) stranger non-mahram applies to a slave is weak argument. Because it is established that a slave is different from a free person as evident in the Quran 2:178, 4:25, 16:75, 33:55 and in several ahadith.