Open organizers pay out their prize money in dollars, but the European Tour distributes it using details in their database.
Fleetwood has finished no worse than 17th in this year's three major championships to date, most famously carding a record-equalling final round of 63 in June's US Open to finish just a shot behind the victor, Brooks Koepka.
Tommy Fleetwood has been one of the hottest golfers of 2018, with five top-10 finishes 14 events played.
The accidental recipient of the $154,000 accounting error was with his friend Greg Thorner when he made the discovery, who shared the story with Twitter promptly after realizing what had happened.
Tommy Fleetwood is no stranger to large checks considering his PGA Tour and European Tour winnings, but Thomas Fleetwood in Clermont, Florida is.
The "other" Tommy Fleetwood, in St. Louis this week beginning the 100th PGA Championship, said he didn't hold any ill toward the European Tour and was able to smile about the story.
With love from The European Tour.
"At least, he can always say he won $150,000 at the Open".
"It was an honest mistake", Fleetwood said, sounding dumbfounded that he had suddenly become a focus of media attention.
Some of the accounting confusion may be linked to the fact that the elder, less hirsute Fleetwood also was an accomplished golfer in his youth, played in tournaments in Europe and then tried to play on the European Tour's senior circuit, Golf.com reported. It's a amusing story.
In that case, he probably won't mind if non-Open Tommy Fleetwood just holds onto the cash.