Does a wronged person's waiver of their right against the oppressor invalidate the answering of their prayer against him when they are subsequently harmed by that injustice?

After forgiving an injustice and relinquishing his right, a Muslim should not go back on his word or supplicate against the one he forgave, for he who goes back on his gift is like a dog that returns to its vomit. Forgiveness is among the greatest acts of worship, and he who forgives, his reward is with Allah. And Allah does not increase a servant in forgiveness except in honor.

Patience and seeking reward from Allah Almighty are required. Supplicating against an oppressor after forgiving him is an act of aggression in supplication, which is forbidden, unless the forgiveness was conditional upon something that was not fulfilled.