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.
It is permissible to 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