All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The complaint about the boss or the manager if it is an injustice committed by him to the employee, then the view that we hold here at Islamweb is that if the victim speaks about the injustice of the transgressor in front of others as a way of clarifying his injustice or warning others against him, then there is no sin on him, and it is not considered forbidden backbiting, especially if the oppressor does not accept advice. The evidence about this is that Allah says (what means): {Allah does not like the public mention of evil except by one who has been wronged. And ever is Allah Hearing and Knowing.} [Quran 4:148]
The second evidence is that the Prophet said: “‘Delay in payment on the part of one who possesses the means makes it lawful to dishonor and punish him.”
Hence, the jurists have excluded complaining about injustice from the forbidden backbiting.
But if it is just a behavior or interaction that the employee does not like, then this is backbiting that is not permissible. It is well known to the Muslim that backbiting is forbidden, and it is a major sin. Talking about someone’s behavior in a manner that that person hates, then there is no doubt that this is considered backbiting, because the Prophet has profoundly explained backbiting as he said: “Saying something about your brother that he hates to be said about him.” [Muslim]
An-Nawawi said in al-Athkaar:
“Backbiting is saying something about a person which he hates to be said about him; this is whether you mention him verbally, or in your book, or that you wink to people about him with your eyes, or point at him with your hand, or your head. Everything that you make others understand that there is a deficiency in the Muslim, it is forbidden backbiting.” [End of quote]
For more benefit on when backbiting is permitted, please refer to Fatwa 82663.
For more benefit on whether or not complaining about a person's injustice constitutes backbiting, please refer to Fatwa 367414.
Allah Knows best.