Considering the language barrier, a slap on the wrist is less appropriate. Facebook could have used this opportunity to publicize explaining to Khalil that his bug finding techniques are not in accordance with the guidelines and garner good will by paying him the $500 as an exception to the rule. The media would be frantically covering how facebook in spite of its guidlines decided to thank the person who reported the bug and overlook an apparently innocent mistake. A missed opportunity on facebook's end.