How can the non-mitigation of punishment for a thief by invoking a curse upon him be consistent with the Hadith: "Whoever invokes a curse upon his oppressor has triumphed," and why is mere invocation considered a triumph while true triumph lies in the oppressor receiving punishment?

The oppressed is entitled to take from the good deeds of the oppressor on the Day of Judgment. If the oppressed supplicates against the oppressor in this world, then he has taken some of his right and lightened the burden on the oppressor. There is no sin upon the oppressed for this, but he misses out on the reward of forgiveness. Forgiveness has ranks: the highest is forgiveness itself, then seeking justice, like supplication, and the lowest is transgression against the oppressor. Whoever supplicates against his oppressor has sought victory for himself, and whoever forgives his oppressor for the sake of Allah, Allah will honor him.