How can God command His servants not to despair of His mercy, while He caused Abu Lahab to despair of His mercy in His saying, the Almighty: "May the hands of Abu Lahab be ruined, and ruined is he"? And is it permissible for God to cause a servant to despair of His mercy?

Allah does not cause people to despair of His mercy because we do not know the unseen, and judgment is not ours; rather, creation and command belong to Allah. He forgives whom He wills and punishes whom He wills. Allah has decreed despair of His mercy and deserving of punishment for disbelievers. So, Allah causes a servant to despair of His mercy if that servant deserves it due to rejecting the truth and turning away from it after it became clear to him. This is from Allah’s justice and wisdom. People are of two kinds: a guided group, who do not despair of Allah's mercy, and a group upon whom misguidance is due, who are the people of despair and hopelessness.