What is the ruling on a person testifying, without having witnessed the incident, that the officer slandered the accused, and is such testimony considered false, even if it is to save the accused and no harm befalls anyone?

The principle is that false testimony is forbidden, unless it is to avert a significant harm that cannot be avoided except through it. In such a case, lying to avert harm is permissible if you are certain of your uncle's innocence, based on information from reliable sources. However, if you do not know the truth of his situation, or if you know that he was caught with drugs, then testifying on his behalf is not permissible. Testimony is only permissible for what you know and are certain of its truth, based on observation or reliable hearing; as Allah states: ﴿إِلَّا مَنْ شَهِدَ بِالْحَقِّ وَهُمْ يَعْلَمُونَ﴾ [“except for those who testify to the truth, and they know”] and His statement: ﴿وَمَا شَهِدْنَا إِلَّا بِمَا عَلِمْنَا﴾ [“and we did not testify except to what we knew”].