The man had left his car at the shop to undergo repairs and had been given a temporary replacement car to use while the work was being done, the Göteborgs-Posten (GP) newspaper reports.
As he drove off in the borrowed car, the man noticed a black and beige-coloured back in the back seat.
Curious, the man opened the bag to inspect its contents.
Much to his surprise, he discovered the bag was filled with neatly bundled and carefully packed stacks of cash.
“There were packs of 100 kronor notes in plastic bags. It’s not every day you find several hundred thousand kronor,” the man told GP.
All told, the mysterious bag contained around 500,000 kronor ($66,000).
Instead of making off with the cash, however, the man returned to the repair shop and reported his remarkable find.
The mechanic then got in touch with the man who had borrowed the car the day before, the owner of a Gothenburg club, who hadn’t yet had a chance to report his missing bag.
The thankful club owner told the man that his good deed wouldn’t go unrewarded.
“He wants to compensate me somehow, but we haven’t agreed yet on how,” said the man.