Is there a sin upon me because I did not give a person 5 Riyals that I had in my wallet, even though I told him I did not have them with me, so he gave me 20 Riyals instead of 15 Riyals that he owed me, and I cannot find him now? What is the ruling on that?

If you deliberately informed the questioner that you had nothing, while knowing that there were five Riyals in your wallet, then this is a lie from which you must repent. Lying is one of the major sins.

However, if you only meant that there was nothing in your pocket, then this is a dissimulation (tawriyah), which is prohibited here because it is a means to take what is not rightfully yours.

If the person left the five Riyals for you willingly, then they are permissible for you. Otherwise, you must ask him in order to give him his right. If he cannot be reached, then give them in charity on his behalf with the intention of guaranteeing them.