The right belief and expression is that it is "intrinsically/absolutely impossible", and not contingent upon anything.
It should be clear that the meaning of "contingently impossible" is "intrinsically possible". It is just that the latter expression is not as easy to sell to ignorant imitators in belief.
1) Aļļaah’s Eternally Speech (which is not letters, sounds, or language) must be true, and cannot be untrue, because it is not created. In a speech that could tell a lie, a specific lie is only one possibility among infinite possibilities, thus such a speech would need to be specified and brought into existence, i.e. it would have to be created. This is unlike true speech, because the truth can only be one about any particular matter, and is known by Aļļaah eternally. This is the meaning of imam Ahmad’s saying, "His Speech is from His knowledge," i.e. "agrees with His knowledge," and His knowledge is One, Eternal, and True just as His Speech.
2) The uttered speech that is called "Aļļaah’s Speech" tells us what He said with His Eternal Speech. The Qur’aan in the sense of the book with Arabic expressions is utterable speech that tells us what Aļļaah said eternally. That is why it is called Aļļaah’s speech, even though Aļļaah’s actual attribute of speech is eternal, and is not letters, sounds, or language. This has been extensively explained in "The Qur’aan and Aļļaah’s attribute of Speech."
3) According to 2), an utterance that says something other than what Aļļaah said eternally is not His Speech.
4) Therefore, any uttered speech that is not true cannot rationally be said to be Aļļaah’s Speech, because it does not tell us what He said eternally.
5) Conclusion: it is impossible in the mind’s eye that Aļļaah’s so called kalaam lafţħiyy (speech of utterable expressions) could contain a lie.
As for what Al-Jurjaaniyy, Al-Iijiyy and At-Taftaazaaniyy are saying; what they mean is that the speech that we say refers to Aļļaah’s eternal Speech, how do we prove that it really is true, i.e really refers to Aļļaah’s eternal speech? The proof for that is one of normal necessity according to some scholars, that is, the miracles of the Prophet, the agreement of everything in the scripture with the truth, and so on prove that. They don’t mean that it is contingently possible for Aļļaah to lie, as is clear from the context.
That being said, it is important that one does not read books, other than the Qur’aan, assuming that every letter in the book was actually written by the author. There could be perversions, and there could be slips of the pen. It happens all the time. This is in addition to the fact that there is no proof in what a non-Prophet says, especially in belief issues. The sources of knowledge according to Sunnis is either observation, or pure reason (not depending on repeated experiences, but coming from a valid conclusion necessitated by irrefutable premises), or information from a prophet.