That guy didn't even post an paid ad---he posted it on craigslist.
Last week, tired of arguing with this verbally abusive guy, my friend even orally offered a cut of $100 from the half month's rent, so that guy could give back security deposit+half month's rent less $100. This guy, being totally greedy, would not accept and wanted to swallow the half month's rent.
Thank you so much for your replies!!