Is it permissible for me to benefit from the 500 Riyals that a boy found in the street and gave to me to deliver to the Imam, after I searched for its owner for a year and did not find him, or should I return it to the boy who found it?

If the finder delayed announcing the lost item for about a month, he must compensate for that in the following year. You were remiss in announcing the lost item by limiting yourself to social media, whereas you should have advertised it more comprehensively, such as by hanging signs or calling out for its owner. Since you delayed and were remiss in announcing it until a year passed, scholars have differed on whether the obligation to announce it lapses and it enters the finder's possession. If it is determined that it has entered his possession, then you must return it to the guardian of the boy, because he is the one who found it.