Is the saying, "Appearance is costly, and demand is cheap," when interpreting the verse, "So when his Lord appeared to the mountain, He rendered it level," considered disbelief or an insult to the interpretation of the noble verse?

The saying "precious yet cheap to acquire" does not imply underestimating what is sought, but rather the ease of its attainment.

As for the phrase "Allah's little finger," it is incorrect, as it is not mentioned in any sacred texts. The mention of the little finger (khinsar) was to illustrate the smallness of the extent to which Allah manifested Himself to the mountain, and Allah's greatness that crushed the mountain with such a slight manifestation. Ibn Abbas's statement, "He did not manifest except like the little finger," is a metaphor for belittling something.

As for distinguishing between tajalli (manifestation) and dhuhur (appearance), there is no source for it in books of tafsir (exegesis) or language. Tajalli is dhuhur, and this is the view of the Ahl al-Sunnah wal Jama'ah and all linguists.