French police found the man late Saturday morning next to the road to Biesheim in the Alsace region of eastern France, and took him to hospital for a check-up.
The man in his 40s is suspected of involvement in cases of fraud, police said, but would not provide further details of the cross-border escapade.
On the German side of the Rhine, police were questioning three French nationals - two men and a woman - who were seen with the kidnapped man near Freiburg.
Information from the investigation suggests victims of the alleged crook decided to nab the suspect themselves and turn him over to the authorities.
He will soon face court regarding the arrest warrant against him.