He made sure it was returned to the forgetful owner.
"I don't care from $10 to higher. My dignity is not for sale, and that's the way I am. That's the way our parents taught us when I was a kid," cabbie Gerardo Gamboa told CNN affiliate KVVU. "All I wanted to do was just call my dispatcher and return it, and this is the only chance I have to show cab drivers in Las Vegas are not bad people."
The loser of the money -- at least temporarily, thanks to Gamboa's good deed -- was a famous 28-year-old professional poker player, whom officials weren't publicly identifying, according to another CNN affiliate, KLAS.
The gambler was so devastated at his loss Monday that "he wanted to shoot himself in the head," a cab company official told KLAS.
"He was grateful. He was very understanding; he wasn't irritated. To a point he was irritated, but he wasn't mad or aggressive or anything. Maybe a little more embarrassed than anything. I mean, he left $300,000 in a cab, but he was an awesome guy," Yellow Checker Star field operations manager Joel Willden told KVVU.
The chief operating officer of the cab company described the sum as the most found in a cab in his 32 years in the business.
"We had to be as a company extremely cautious that the right person got the money," cab firm CEO Bill Shranko told KLAS. "It's the Christmas season and we think it's the best story that I've seen in 32 years, and cab drivers worldwide are going to be very proud."
It all began when Gamboa drove a man about two miles between casino-hotels, from the Cosmopolitan on the Las Vegas Strip to Palms Place, KLAS reported. The man gave him a $5 tip.
Gamboa then went back to the Strip -- the Bellagio casino-hotel. "When the doorman at the Bellagio opened up the door for the next passenger, he said, 'There's a bag in here. It's full of chocolate,'" Gamboa told KLAS.
When he opened the bag, Gamboa was stunned at the six bundles of $100 bills inside. His next fare was already in the cab, and Gamboa told the passenger, "Look sir, you are my witness. I didn't touch anything here," Gamboa told KLAS.
Meanwhile, the poker player went to the cab company looking for his lost money.
Gamboa took the small fortune to the office of his Nevada cab company, which contacted authorities. After a four-hour police investigation, officials confirmed the gambler was the rightful owner of the money, KLAS reported.
Yellow Check Star gave Gamboa a $1,000 reward, made him driver of the year, and gave him a steak dinner for two, the affiliates reported.
"I did the right thing, that's it," Gamboa told KLAS. "They call us sin city. But it's not. It's angel city."