كَلِمَةَ حَقٍّ means a statement that is true and just. Correcting someone in authority requires reliable facts. Anger, exaggeration, or an unverified accusation does not become truth merely because it is expressed boldly.
For example, when a decision unfairly denies a vulnerable person a right, presenting the facts accurately and respectfully serves justice. Prudence, civility, and attention to consequences are part of responsible conduct.
In مَنْ قَالَ, مَنْ refers to the person who acts and قَالَ is a past-tense verb, “he said.” The value highlighted is the truth of the statement, not the social position of the speaker.