The Second Verse: [“And those who have disbelieved say, 'You are not a messenger.' Say, 'Sufficient is Allah as a witness between me and you, and [so is] whoever has knowledge of the Scripture.'”] 1
The relative pronoun man (“whoever” or “who”) is an explicative apposition ('atf bayan) or a substitute (badal) for the word of majesty “Allah” preceding it. That is to say: Sufficient is Allah, who has the knowledge of the Book, as a witness between me and you.
[75] And it has been reported in the book Yanabi' al-Mawaddah from Abu Sa'id al-Khudri (Allah was pleased with him) who said: “I asked the Messenger of Allah—Exalted is His might—about this verse:['Said one who had knowledge from the Scripture, "I will bring it to you before your glance returns to you."'] 2 He said: 'This is the legatee (wasi) of my brother Sulayman bin Dawud—peace is upon them both—meaning Asif.' And I asked him about the saying of Allah—Mighty and Majestic: [“And those who have disbelieved say, 'You are not a messenger.'