Ta'anis 2:1: It does not say that He saw their sackcloth and fasting rather, their deeds, that they repented.

Malbim: Initially, their Teshuvah was mere regret, but they did not repent in deed to fix the past. After the king's command, Hashem saw their deeds - they repented in deed and returned the theft.

Radak: They repented fully, also from theft. One who stole a beam and built it into a building, he takes apart the building and returns the beam to its owner (Gitin 55a).

Malbim: They returned theft, but they did not repent from idolatry. Since the decree was due to theft, and it was via Hashgachah, it was canceled. If evil was due to come based on the Ma'arachah, Hashem would not override nature for the sake of idolaters.

Rashi: He changed His thoughts about the evil decree, and retracted from it.

Radak: All His words to harm people are on condition that they do not repent. If they will repent, He will pardon - this is one of His Midos, like it says in the Torah ¹ , and in Yirmeyah "Rega Adaber Al Goy... Linsosh v'Linsotz...; v'Shav ha'Goy ha'Hu..." (18:7-8) and in Yechezkel "uv'Shuv Rasha me'Rish'aso... Es Nafsho Yechayeh" (18:27).