What is the ruling on exposing a slanderer to reveal his lies and ward off an accusation from an innocent person, such as by revealing a recorded call in his voice that proves his slander?

The general principle is to conceal a Muslim's faults and advise him secretly. Then, the slanderer should be commanded to rectify the damage he caused. If he does not take heed and stop, and if warding off injustice from the slandered person can only be achieved by exposing the slanderer's affair, then it is permissible to expose him in order to restore rights.