German authorities claim the former PetroVietnam Construction Company (PVC) chief was abducted by Vietnam's secret service and has publicly condemned the kidnapping.
However, in an interview broadcast on state media Thursday, Trinh Xuan Thanh said he had decided to return home to answer questions for alleged financial crimes.
"During my time as a fugitive, I realized that what I had (done was) wrong. I did not think carefully. I realized that I had to return to face the truth, to admit my mistakes. My family encouraged me a lot," he said.
German authorities first announced on Wednesday an asylum seeker had been taken from Berlin back to Vietnam against his will.
The Vietnamese ambassador was summoned to meet the state secretary of Germany's Federal Foreign Office, Markus Ederer, on the same day to address the allegations.
"The abduction of Vietnamese citizen Trinh Xuan Thanh on German territory is an unprecedented and blatant violation of German and international law," Germany's Foreign Office said in a statement.
Vietnam's Foreign Ministry said they regretted Germany's accusations against their country.
"Vietnam very much respects and wants to develop the strategic partnership between Vietnam and Germany," foreign ministry spokeswoman Le Thi Thu Hang told reporters according to Reuters.
According to CNN affiliate and Vietnamese state broadcaster VTV, Thanh "gave himself up" to Vietnam's Investigation Security Agency out of anxiety over the crimes he committed.
Thanh is a former state oil executive who has been accused by the Vietnamese government
of "alleged violations of state regulations on economic management" which resulted in a loss of nearly US$142 million for PVC.
"I was most responsible for the financial losses of PetroVietnam Construction Company (PVC). Out of such fear, I decided to stay in Germany. I was tired of that fugitive life," VTV quoted him as saying.
Vietnamese spy ordered to leave
In retaliation, Germany ordered the official representative of Vietnam's intelligence services to get out of the country by Friday night.
"We also reserve the right to take further steps as required at the political, economic and development policy level," the Foreign Policy statement said.
Ederer called on the Vietnamese government to allow Thanh to return so he could then be considered for extradition through the proper diplomatic channels.
In a weekly government press conference in Berlin, German's foreign ministry spokesman Martin Schäfer reiterated his country's condemnation of Vietnam's actions.
"The kidnapping of Vietnamese citizen Trinh Xuan Thanh on German soil is an unprecedented and scandalous violation of German and international law," Schäfer told reporters.