The friend refused. No no no, he said. That’s not how you invest in a tech startup.

Yes, this is actually what happened when John Shahidi asked Floyd Mayweather if he’d invest in Shahidi’s selfie app, Shots: Mayweather agreed, then tried to hand over most of his investment in cold cash, because, after all, he happened to have quite a lot of it with him, in a bag. (He had just won a large bet on a Philadelphia Eagles game.) But Shahidi had to turn down Mayweather, who for all his earnings (he makes, on average, a minimum of $32 million for one boxing match; sometimes as much as $70 million thanks to his cut of the Pay-Per-View returns) may not have a great deal of experience with more formal investment procedures and board-room meetings. “Champ, we can’t do it that way,” Shahidi recalls telling Mayweather. “There has to be paperwork.”

Mayweather, who is number one (for the third time in a row) on our 2014 Fortunate 50 list of the highest-earning U.S. athletes and is set to earn a conservatively estimated $105 million this year, has taken his investment in Shots seriously. He constantly tweets links to the selfies he takes using the mobile application– and he uses it often. He even used the app to announce the news of his May 3 fight against Marcos Maidana.

The other big-name investor in Shots is the pop star Justin Bieber (as we reported exclusively in November), who is a friend of Mayweather’s and has accompanied the boxer on his arena entrance walk twice in a row. Bieber’s involvement in funding the app, unsurprisingly, has attracted most of the headlines; very few of the press stories about Shots have even mentioned Mayweather, even though he invested earlier than Bieber. (Bieber has invested more money in Shots than Mayweather, but both have invested just over $1 million.) For Bieber, the app investment marked his first without the guidance of his business manager Scooter Braun. It is the first time Mayweather has invested in any technology.

Originally called Shots of Me (the public URL of Shots photos is shots.me), the app has struggled to gain a big foothold, though it is popular with a slew of boldfaced names including Shaquille O’Neal, Snoop Dogg and the Buffalo Bills running back C.J. Spiller. It has also been a punching bag of the tech blog Valleywag, which called it a “total dud.” Shahidi said that he has dealt with negative coverage by asking Mayweather how he deals with bad press.

Shahidi met Mayweather in 2010 and soon began handling his marketing efforts. Shahidi’s company, RockLive, had started that year as an iOS game developer, releasing mobile games with athletes like the star soccer player Cristiano Ronaldo and American football player Chad “Ochocinco” Johnson (the company’s first game: Mad Chad). One of the games they had created, Mike Tyson’s Main Event—a sort of update on the classic Mike Tyson’s Punch-Out!!—managed 1 million downloads in its first week and, for a time, was the number one mobile game in the U.K. and number three mobile game in the U.S. But when Shahidi came up with the idea for Shots, he pulled all of the games because RockLive didn’t have enough staff to keep them going while also working on Shots. RockLive rebranded and became Shots Mobile in August 2013.

In 2012, after Shahidi had helped Mayweather set up his Twitter and Facebook accounts and guided him on social media, he told the fighter about Shots. Mayweather recalls, “I loved the idea. He told me, ‘We’ve built it but need to release it at the right time.’ Something gave me a good feeling about what he was saying and I felt like Shots was going to be something big.”

There are two goals guiding Shots, Shahidi says. The first is to reduce bullying, which Shots accomplishes by not allowing any comments whatsoever—you can like a photo, and reply to it with a photo of your own, but you cannot comment. Shahidi says this addresses the very real problem of bullying among teens: Middle-school girls, for example, can’t post judgmental comments on Shots the way they can on, say, Instagram. Shahidi says this was one element that particularly appealed to Mayweather as soon as he showed the fighter the kind of cruel comments (you may know it as “trolling”) that people were posting on the public Instagram photos of Mayweather’s daughter Iyanna.

The second goal is to encourage people to be themselves, flaws and all. (And what is a selfie if not a celebration of the self?) For this reason, the app has no filters and only allows the posting of photos that are taken right now; unlike Instagram, it does not allow you to upload photos snapped earlier or from another program. The point is so that everything feels more “in the moment,” Shahidi says.

Despite Shahidi’s best intentions, the app’s position has fluctuated wildly in the U.S. App Store rankings since its debut last November. Its best-ever ranking, according to TopAppCharts.com, was #10 in the Photo & Video category. On Monday it was ranked #82 in Photo & Video. But the app has been downloaded more than a million times, Shots says, and currently is focused on “enhancing the product and expanding to more users” rather than worrying about generating revenue.

Mayweather tells Fortune that he feels optimistic about the app, and that being involved has piqued his interest in doing additional tech investing. He also stresses that he believes in the app’s vision and ethics. But what is clear is that it is also Shahidi, more than just Shots, who Mayweather has put his faith in and who will guide his friend’s future involvement in the space. “I own buildings, concert promotion companies, music publishing and other long term businesses, but Shots is my only tech investment,” he says. “I want to fully understand a business before investing . . . I am going to talk to John and see what other cool non-competing startups might interest me. It’s a great area to invest in and I trust John’s knowledge of the industry to help me.”

Though he currently carries a perfect boxing record of 46-0, Mayweather is nonetheless looking toward the day when he might retire from the ring. And he’s seeking ways to ensure that the money keeps on coming in. “I’m building for my post-fighting career,” he says. “And the long-term thinking behind Shots fits.”