A Rat Walkthrough
After completing the task from Katya, go to Kovalev. After talking to him, you will find out that there is a rat in the shelter, in the city. After this conversation, go to a bald man and talk to him about the rat. Remember not to accuse him right away, but simply get information about his identity from him. Return to Kovalev with new info. You should get money for just getting the information.
Return to the bald man and tell him the truth. Follow him to an abandoned hut outside the city, beat him up there and take his belongings.
Then talk to Kovalev again and you will get the next part of the reward.