Mafia III: Definitive Edition

Basically, you get 2% of whatever your Underboss' current earn is, so, what you need to do is get one of your Underboss' current earn to $500,000.
The first three starting districts (Delray Hollow, River Row, and Pointe Verdun) give $100,000 when the rackets are maxed from the delivery side missions.
The Lieutenant's districts (Tickfaw Harbor, Downtown, and Barclay Mills) give $120,000.
The Capo's districts (Frisco Fields, French Ward, and Southdowns) give $140,000.
The fastest way is you need to give one of your Underbosses: starting area, a Lieutenant's area, and two Capo's areas.
I recommend going for this on your keep one Underboss alive playthrough, unlike me and you have to do three playthroughs.
I really hope my first guide helps someone, cheers.

In order to Unlock this achievement you must increase the earn of at least One Underboss to 500k so that you receive kickback of $10k. Check below for method if you are assigning districts evenly.

Note: This is not awarded for saving up your kickback and then collecting it when it reaches 10k. You must receive a one time kickback of at least 10k

There are various methods of increasing earn. The higher the earn the higher the kickback.

When you take over a racket you also receive bonus earn for sparing informants and racket bosses. This is a massive help and can increase the earn of one racket from 50k to 70k.

Simply taking over a racket does not mean you will earn the maximum earn for that racket. Load up the map, hover over the racket and it will tell you how much that racket is making and what the maximum is. Eg if you are earning 50k but the maximum is 70k then you will need to a side mission which permanantly increases the earn of a racket. For Cassandra the side Mission is "Are We Cool", Burke "The Righteously Fucked" & Vito "Seems Simple Enough".

If you are evenly distributing the districts amongst the underbosses then you can still earn this achievement by completing the following method.

The Method
This method will only work if you have been sparing and recruiting informants/racket bosses.

When you reach the final district in the game you should have two underbosses who control 3 districts each & One Underboss who controls two districts.

When you begin taking over rackets in the final district assign them instead to an underboss who already controls 3 districts, preferably the underboss with the highest earn. You only need to assign the rackets NOT the district as a whole.

Now begin completing the Racket side Missions. I chose Burke which required me deliver Trucks of Booze to his rackets. Scroll over the rackets of your underboss and whichever has the lowest earn then make sure you make the delievery to that racket. (Usually every 20k delivery will increase your kickback by $200-300.

The three districts I assigned to Burke were Frisco Fields, Point Verdun & Tickfaw Harbor
The two rackets I assigned him were in French ward (However I did not assign him this district).

Once all of the rackets were at max earn the kickback was exactly 10k.
When the Kickback reaches 10k you then need to wait for a payment, once it has been collected the achievement will pop.

Now Complete the final mission of the district and at the sit down assign the district to your underboss with 2, so that each has three. The rackets will then be taken off your chosen underboss so make sure that you have the achievement.

Tip: Rather than completing the racket side missions in one sitting, chose an Underboss early on who you are going to max out, then beginning do one of the racket missions whenever you have a chance. Saves a lot of boredom