مَا يُبَدَّلُ الْقَوْلُ لَدَيَّ
The word is not changed in My presence. (Qāf 50:29)
The objection appeals to the unalterability of divine speech: how could a punishment be left unexecuted? The response argues that the binding content is what the statement truly asserts. Recognizing an exception to a general warning is not the same as falsifying a categorical divine prediction.
The issue is the exact scope of the text. If pardon is a valid instance of qualification, it does not amount to a change in what Allah has truly declared.