What is the expiation for someone who, in a fit of rage over a worldly matter, threw the Mus'haf from his hand, and is this considered an unforgivable sin, especially with a strong feeling of remorse?

Throwing the Mus'haf (copy of the Qur’an) with the intention of disrespecting it is an act of apostasy and disbelief (kufr), and requires sincere repentance. However, if it was not done with the intention of disrespect, it is not considered disbelief, but rather a heinous act. Nevertheless, since you have regretted it and repented, we hope that Allah accepts your repentance and forgives your sin, based on His saying: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful'" (Quran 39:53), and based on the Prophet's (peace be upon him) sayings: "The one who repents from sin is like one who has no sin," and "All of the children of Adam are sinners, and the best of sinners are those who repent," and "Regret is repentance."