"The kidnapping of Vietnamese national Trinh Xuan Thanh on German soil is an unprecedented and scandalous violation of German and international law," said foreign ministry spokesman Martin Schäfer, accusing Vietnam's secret service and embassy of snatching the man.
"As a consequence of this completely unacceptable action, we are declaring the representative of the Vietnamese secret service at the embassy persona non grata, and giving him 48 hours to leave," added the spokesman.
The foreign ministry had summoned Vietnam's ambassador over the affair, said Schäfer, to underline that "this sort of action has the potential to seriously damage German and Vietnamese ties."
According to German media, the asylum seeker was snatched on July 23rd in central Berlin's Tiergarten park - next to the Bundestag (German parliament) and the Chancellery - by armed men.
The man had resurfaced on Monday in Vietnam, where state-owned newspapers reported that he had turned himself in to authorities.
Thanh headed the state-owned Petro Vietnam Construction Corporation but had been sought by Hanoi. Once a lawmaker, he was stripped of his seat and kicked out of the communist party.
According to German news agency DPA, he has been accused of having caused losses amounting to €125 million at the Vietnamese company.
Schäfer said Hanoi's actions marked a "breach of trust" as talks had been underway during the G20 summit of world leaders in Hamburg about a "possible extradition in a legal manner".
"We demand that Trinh Xuan Thanh be allowed to travel back to Germany without delay, so that Vietnam's request for extradition and his request for asylum can be thoroughly examined," said Schäfer.