Mafia III

This achievement, as well as "Cash in Hand," is cumulative across both playthroughs. Money is collected and earned in a variety of ways: completing story missions and side missions, collecting Racket kickback earnings, looting bundles of cash in enemy-occupied territory (marked with a $ icon on the mini-map), and picking up fallen wads of cash after killing enemies.

To increase the total potential earnings from your Underbosses, you will want to complete all of their side missions when they become available, assign them all new rackets, wiretap rackets to recruit Racket Bosses, and recruit Racket Informants.

Lastly, it's worth mentioning that when you die you lose 50% of the money in your wallet. It may be wise then to store money in the safe at Sammy's old place. Once you rescue Vito and unlock his "Consigliere" Associate, you can use this service in the weapon wheel to call Betty and have her come pick up your money for safekeeping.