How can one repent from backbiting 150 million people, and is it obligatory to apologize to all of them? And how can a debt owed to a club's guard, whose trace has vanished, be settled?

A Muslim must guard his tongue from slandering the honor of other Muslims, as the Prophet, peace and blessings be upon him, said: "Every Muslim's blood, wealth, and honor are sacred to another Muslim." What is mentioned in the question is not backbiting, because backbiting only occurs when referring to known individuals. So, if someone backbites the people of a village, it is not considered backbiting until they name specific, known individuals. And it is not appropriate to generalize in this manner, even if one intends to particularize.

As for the remaining Riyals, if they belong to the club, then pay them to the club. If they belong to the guard and you are unable to meet him, then give them as charity on his behalf for the general welfare of Muslims, based on the statement of Shaykh al-Islam Ibn Taymiyyah regarding someone who despairs of knowing the owners of funds: "He should give it as charity on their behalf, or spend it on the welfare of Muslims, or hand it over to a just distributor who will spend it on legitimate Islamic interests."