Compound Interest (Raf) is one of the Mr. Faber Work missions in GTA Online that arrived with the Money Fronts update. You can access it either through the laptop in the Hands on Car Wash or the Smoke on the Water business. You must first purchase these businesses and set them up to complete various tasks for Mr. Faber and make money.
This article offers a brief guide for everything you should know to complete this mission.
A brief guide to GTA Online Compound Interest (Raf) mission
Where and how to start the mission

There are some requirements that players must complete if they wish to start the Compound Interest (Raf) task. The first one is to purchase the Car Wash business - . This will cost you $1 million and becomes a great way to make passive income in the game. Next, you must purchase the Smoke on the Water business, which is a pharmacy that deals in weed.
This required you to spend $850,000 and visit the shop at Vespucci Beach. Note that Smoke on the Water has both legal and illegal missions. Doing the illegal ones fills the Heat Bar, and the more heat you have, the more trouble you will face.
To start the Compound Interest (Raf) mission, access the laptop in the Car Wash office and select Mr. Faber Work. You will eventually trigger the Compound Interest mission (we will update the right order of the Mr. Faber Work missions shortly).
How to complete the mission

When playing the Mr. Faber Work missions for the first time, their difficulty is set to Hard by default. However, the missions aren't too challenging, so most players shouldn't have any issue completing them. Once you confirm the settings and select the loadout, it is time to head out.
After teleporting outside the building, you can pick up one of the Money Fronts vehicles and reach El Burro Heights. It will be marked on your map, and you can just follow the designated path. Your next task will be to reach Skinny's house, who is targeted by the Triads. Once you reach the house, you must defeat all the Triad members and head inside.

Now, search the house to find Skinny and talk to him. Your next task will be to take him to the lockup. Note that you must again fight a couple of Los Santos Triad members once you reach the container. Skinny will then open the container, inside which you will find the diamonds taken from the Casino.
The final task is to return to the Casino and hand the diamonds to the other party. Doing so will complete the Compound Interest (Raf) mission and give you XP and money.
Also check: Five major things added to GTA Online with the Money Fronts update