The girl was seven months old when they had to return to Bulgaria, and she said she could not afford to take the child with her.
She said she was talking to a woman she worked with who told her: "Give me this child, I will take care of it. You can come and take it back anytime you want.
"But I had other children to take care of and I couldn't go... I have never got any money for it."
Bulgarian officials said that during questioning, Ms Ruseva said she recognised Christos Salis and Eleftheria Dimopoulou as the people she left her child with.
Prosecutors announced they had pressed preliminary charges against Ms Ruseva for "deliberately selling a child while residing out of the country".
"A DNA test has been taken from Ruseva, and information has been collected about her trips to Greece in the last years,'' said a statement from the prosecutor's office.
Greek news site zougla.gr (
http://tvkosmos.gr/article.asp?newsid=2046984)
earlier published what it said was the identity card of Ms Ruseva and a birth certificate for Maria from a hospital in Lamia, not far from Farsala and Larisa.
The birth certificate says the girl was born in January 2009.
Christos Salis and Eleftheria Dimopoulou told police Maria was four years old, but the Smile of the Child charity say medical examinations suggest she is more like five or six.