How was a Qur’anic verse established in the Mus'haf during the time of ‘Umar, only to disappear later? And why were not all Muslims aware of it despite the public dissemination of the Qur’an?

Allah guaranteed the preservation of the Holy Quran, and the Ummah transmitted it exactly as it was revealed, without addition or omission, despite the doubts raised by enemies. The Quran was revealed in seven ahruf (modes of recitation), and Allah abrogated some verses. Gabriel reviewed the Quran with the Prophet twice in the last year [of his life], and this final review was witnessed by Zayd ibn Thabit and Abdullah ibn Mas'ud.

The compilation of the Quran passed through two stages: the first during the time of Abu Bakr al-Siddiq, out of fear of its loss, and the second during the time of Uthman ibn Affan, to unite the Ummah upon a single Mushaf based on the final review, with the consensus of the Companions, including Ubayy ibn Ka'b.

As for the hadith "If the son of Adam had two valleys of wealth...", scholars differed on its origin: whether it was a verse whose recitation was abrogated (which is the view of the majority), or a hadith qudsi, or a prophetic saying that some Companions mistook for a verse. The narrations of Abu Musa al-Ash'ari, Zayd ibn Arqam, Jabir ibn Abdullah, and Buraydah ibn al-Husayb supported its being a verse whose recitation was abrogated.

Meanwhile, some Companions, such as Ibn Abbas and Anas, hesitated about it being a verse until Surah At-Takathur was revealed, at which point they knew it was not part of the Quran. It is authentically reported from Ubayy ibn Ka'b, may Allah be pleased with him, that he used to recite it as part of the Quran, but then he acknowledged the compilation of the Mushaf based on the final review, which abrogated everything else.

As for the narrations that show a disagreement between Umar and Ubayy, may Allah be pleased with them, regarding this verse, all of them are weak in their chain of transmission. And even if they were authentic, Umar knew that Ubayy might not have been informed of the abrogation of some verses. Nevertheless, Ubayy acknowledged the compilation of the Imam Mushaf based on the final review.