Can someone explain Leorio's wager with the prisoner?
You missed some parts. In Chapter 20 of the manga, when Leorio proposed the bet, the prisoner said:


  Prisoner: All right...I'll bet 20 hours...that he really is out cold.


This would mean that Leorio's bet automatically is that he's not out cold. When Leorio was about to let go of the guy playing dead, the prisoner changed his mind.


  Leorio: Look, if he yells out while falling...we'll accept that Kurapika's lost on a foul. You'll get the win. That seem fair to you?
    Prisoner: Yes, quite fair. But I'd like to change my bet. Let me put 40 hours on him waking up.


So, Leorio's bet was changed to the guy not waking up due to this last minute proposition. Therefore, he lost.