Is OTHER wrong in the post ? POST : The manager at my work is an asshole. He is incompetent, misuses his power, sit at his desk all day and is the person who acts friendly in your face but talks shit about you in the back. 
Although opinion widely shared throughout the company, it’s still my opinion. That have being said. 

One times a employee wants to quit so he gave him a one month notice (common here), all according to the rules. 
He wants him to stay longer because of his position and there wasn’t a replacement ride away. The employee is in his right to say no because he wants to start at is new job. 
The manager then threats to give him an bad review to his future employer. 
Because this conversation was private and the employee has no prove he has to agree with his demanding. 

Knowing the person he is I want to secretly record the future conversation i have with him. Just in case. WIBTA?