Is it obligatory to bear responsibility for an amount of 500 Saudi riyals found approximately 20 years ago?

This money rightfully belongs to its owner, and it must be returned to him if he is alive, or to his heirs if he is deceased. If it is impossible to reach him or his heirs, then you must give the money in charity with the intention that the reward is for him. If the owner of the money or his heir appears after the charity has been given, inform him of what you have done. If he approves of it, then that is well and good. If he does not approve of it, then the amount must be returned to him, and the reward for the charity will be yours.