The little girl, thought to be about four years old with blonde hair and green eyes, was found by police on Wednesday in a camp in the central town of Farsala.
DNA testing has shown the child is not related to the couple, who has been arrested.
Speaking to AFP about the girl's biological mother, a lawyer for the Roma couple, Marietta Palavra, said: "We're talking about a woman who could not raise this child and who gave it to the couple in 2009 through a third party shortly after her birth."
"There has been no kidnapping, no robbery, no trafficking," added her colleague Konstantinos Katsavos, who also represents the 40-year-old woman and her husband, 39.
"They did not buy the child."
Voice of Russia, AFP