Chainsaw Man begins with Denji explaining the different body parts he has sold. Some are more vital than others, but why did Denji sell his balls?
Denji decides to sell his balls to help pay back his debt. His father passes away before paying off a debt to the Yakuza, so Denji inherits it. Selling body parts is one of many ways that Denji helps pay off the enormous debt.
It’s not long before Denji realizes that devil hunting is a lucrative career. Still, large “fees” are subtracted from Denji’s devil hunting pay, meaning it is also a mostly fruitless endeavor.
Denji decides to sell his balls
In chapter #1 of Chainsaw Man, Denji counts up the different ways he earns money. The protagonist is horribly in debt, absorbing his late father’s owings to the local Yakuza.
He says that cutting trees earns 60,000 yen a month. The kidney he sells is worth 1.2 million yen, and he sells his right eye for 300,000.
“Sold one of my nuts for… how much did it go for again,” Denji wonders. “Not even 100,000?”
Despite selling off various body parts and working manual labor, Denji reveals that his debt is only brought down to 38,040,000 yen. Denji realizes that devil hunting is a far more lucrative business, with a single devil netting about 300,000 yen.
Denji walks home after disposing of a Tomato Devil, thinking about the hopelessness of his situation. His boss pays him out hundreds of thousands of yen, but reduces the final pay heavily on account of various “bills” and “fees.” A high paying job will only give Denji a meager amount of pay once it’s over.
A chauffeur for Denji’s boss asks why they employ a mere kid as a devil hunter. The boss explains they’re making Denji “pay back his debt to us.” More accurately, the boss says, Denji is paying off the debt of his deceased father.
Denji’s debt problems are solved when he becomes Chainsaw Man and disposes of the Yakuza.
So that’s the answer. Denji decides to sell his balls because he needs to pay off an enormous debt. He inherits the Yakuza debt from his father, and Denji is often manipulated into accepting pay that is far less than what he’s entitled to.