The verse says...

The verse says: “So We sent against them the flood and the locusts and the vermin and the frogs and the blood as distinct signs…” Living creatures are the agents of Allah. Their mission is sometimes to bring mercy, like the spider’s web at the doorway of the cave for the protection of the Messenger of Allah (S). And, it is sometimes a chastisement mission, like that of the swallows, and, in this verse, the mission of frogs and locusts. However, many calamities are often for training.

In any retribution, there is a respite for people to contemplate and repent and return to the Straight Path. Therefore, the Divine retributions come after completing the argument. They saw the divine sign and punishment, yet they acted arrogantly.

invoke your Lord for us by the covenant He has made with you. If you remove the plague from us, we will certainly believe in you, and we will certainly send with you the Children of Israel’.”*** 135 ***. “But when We removed the plague from them until a term which they should reach, behold, they broke (their promise again).”*** The Arabic term /nakθ/ originally means: ‘to untwist a rope’, but later it has been used in the sense of ‘breaking a covenant’, or ‘violating an oath’.

The verse says: “And when the plague fell upon them, they said: ‘O’ Moses! invoke your Lord for us by the covenant He has made with you.

If you remove the plague from us, we will certainly believe in you, and we will certainly send with you the Children of Israel’.” The Qur’ānic term /’ajal/, in this verse, may be the time which Moses (as) appointed for the removal of a calamity, saying that, for example, it would be removed on so and so day or at so and so hour, in order that they might understand that it was a divine retribution and not an accidental happening.

The purpose also maybe that those obstinate people would finally meet the unavoidable Wrath of Allah, but until the appearance of that time and being drowned in the sea, the punishment was temporarily removed.