You are indeed very lucky that you have dealth with an honest guy at the exchange office.What would you do if he just pocketed the 500 riyals and just kept quiet? and by the way,even if you have remembered him and inquired about it to him,he could have easily told you that you are talking to the wrong person,worse still,he could have called the police and complained!
ALHAMDULILAH.Be very careful the next time,especially when it comes to financial issues.