Dua of the one who is suffering from injustice and oppression are Accepted and Answered by Allah

The Messenger of Allah (salAllahu alayhi wasalam) said to Mua’ad Ibn Jabal (radiAllahu anhu), ‘Beware of the supplication of the unjustly treated, because there is no shelter or veil between it (the supplication of the o­ne who is suffering injustice) and Allah (Subhanahu wa Ta’ala)’ [Sahih Al-Bukhari and Muslim]

The prophet (salAllahu alayhi wasalam) declared, ‘Three men whose dua is never rejected (by Allah) are: the fasting person until he breaks his fast (in another narration, when he breaks fast), the just ruler and the o­ne who is oppressed.’
[Ahmad, at-Tirmidhi - Hasan]

In another hadith; The Prophet (salAllahu alayhi wasalam) declared: ‘Three du’as are surely answered: The du’a of the oppressed, the du’a of the traveler, and the du’a of the father/mother (upon their child)’

The o­ne who is suffering injustice is heard by Allah (Subhanahu wa Ta’ala) when he invokes Allah (Subhanahu wa Ta’ala) to retain his rights from the unjust o­ne or oppressor. Allah (Subhanahu wa Ta’ala) has sworn to help the o­ne who is suffering from injustice sooner or later as the Messenger of Allah (salAllahu alayhi wasalam) said.