Ashwaq Haji, 19, says she ran into the man in a German supermarket in February. Traumatised by the encounter, she returned to Iraq the following month.
Like many other Yazidis, she was kidnapped by IS when the jihadists seized swathes of Iraq in the summer of 2014.
In their ancestral region of Sinjar in northwestern Iraq, thousands of Yazidi women were killed or sold off as sex slaves.
The teenager was held from August 3 until October 22 of 2014, when she managed to escape from the home of an Iraqi jihadist using the name Abu Humam who had bought her for $100, she told AFP in the Yazidi shrine of Lalish, north of second city Mosul.
The Yazidis are a Kurdish-speaking religious minority that was brutally persecuted by the jihadists who despise them as heretics.
Under a German government programme for Iraqi refugees, Ashwaq, her mother and a younger brother were resettled in 2015 in Schwaebisch Gmuend, a town near Stuttgart.
Her refuge in Germany, where she took language lessons, was cut short on February 21 when a man called out her name in a supermarket and started talking to her in German.
"He told me he was Abu Humam. I told him I didn't know him, and then he started talking to me in Arabic," she said.