On Sunday Son Nguyen, a tour guide who takes French tourists to Cat Ba Island, mentioned the incident on social media, saying the couple had taken the taxi from Tran Nhat Duat to Cho Gao Street in Hanoi.
A one-kilometer taxi ride in Vietnam usually costs less than $1.
Then, after getting off the taxi, the man discovered he had forgotten his wallet and passport in the car.
The driver did come back to return the wallet and passport but demanded another VND500,000 for them.
The couple paid the money for their belongings, but were frustrated and took a photo of the driver’s car and license plate.
They then told their guide to report the incident to local authorities.
Son said this was the couple's first visit to Vietnam.
Police in Hoan Kiem District where the incident took place said they summoned the taxi driver, a 34-year-old freelance worker, on Monday. They are investigating further.