Tafsir of Al-Hijr 15:84

84. "So that which they used to earn did not avail them."

It did not repel from them what descended upon them, whether that was their carving of sturdy dwellings, or that along with the amassing of wealth and numbers. Rather, they fell down prone [as dead].

The first mā (in fa-mā) is negating, though it also allows for the possibility of being interrogative. As for the second mā, it may be masdariyyah (functioning as an infinitive), or it may be mawṣūlah (a relative pronoun)—which Abu Hayyan deemed the more apparent—with the implied pronoun (‘ā’id) omitted, meaning: "that which they used to earn."

In al-Irshād, it is stated that the fā’ signifies the consequence of the non-availment specifically at the time of the descent of the punishment, according to what they had hoped for, not a negation of availment in an absolute sense, for that is a continuous state. The verse contains a level of mockery toward them that is not hidden.