Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
The right of the oppressed is never lost with Allah, and retribution on the Day of Resurrection occurs in two forms:
First: Retribution before the Sirat (Bridge):
In this stage, the rights of the oppressed are taken from the good deeds of the wrongdoers. If the wrongdoers do not have enough good deeds, they are burdened with some of the sins of those whom they wronged, according to the extent of the injustices committed, as mentioned in the hadith concerning the bankrupt person. The Prophet ﷺ also said:
“Whoever has wronged his brother regarding his honor or anything else, let him seek his pardon from him today…”
Narrated by Al-Bukhari.
Second: Retribution at the Qantarah (Bridge) after the Sirat:
The Prophet ﷺ said:
“When the believers are saved from the Fire, they will be detained at a bridge between Paradise and Hell, where they will settle wrongs that occurred between them in this world. Then, when they have been purified and refined, they will be permitted to enter Paradise.”
Narrated by Al-Bukhari.
The scholars differed concerning the meaning of this settling of wrongs. Some said that the oppressed will take from the good deeds of the wrongdoer. Others said that Allah will compensate the oppressed with something better than what he lost through the injustice and will forgive the wrongdoer through His mercy. Others said that Allah will compensate the oppressed or place forgiveness in the hearts of both parties.
The closest view is that the first retribution is for fulfilling people's rights, while the retribution at the Qantarah also serves to purify the hearts and remove whatever resentment and hatred may remain within them, so that the people of Paradise enter it with nothing against one another in their hearts.
Allah تعالى says:
﴿وَنَزَعْنَا مَا فِي صُدُورِهِمْ مِنْ غِلٍّ إِخْوَانًا عَلَىٰ سُرُرٍ مُتَقَابِلِينَ﴾ [Al-Hijr: 47].
“We will remove whatever rancor is within their hearts; they will be brothers, seated facing one another on couches.”
Therefore, saying that Allah may forgive the wrongdoer does not mean that the right of the oppressed is lost. Rather, the right of the oppressed is preserved. Allah is far too just to wrong anyone, and He may combine for the oppressed the restoration of his right, compensation for what he suffered, and the removal of the effects of injustice from his heart.
So be reassured, O oppressed person, and do not think that the injustice inflicted upon you will be wasted. Allah does not allow the rights of His servants to be lost, and the justice and bounty that He possesses are greater than anything a human being can imagine.
And Allah knows best.