The girl was just 13 when her father 'sold' her to another man for €15,000, Italy's national police force said on Thursday.
The second man, who lived in France, had already paid €4,000 ($4,770) of a bridal price of €15,000 ($18,000), and under the agreement, the girl was to be taken to live with him this month.
But police were able to rescue the girl from the basement in Florence where her father had kept her, and saved her from "a probable life of slavery", according to a police report.
The girl's movements had been strictly controlled by her father; she was deprived of money and a SIM card for her phone, and was only allowed to leave the house if accompanied by a male member of the family.
Furthermore, the father had promised the 'buyer' that his daughter would maintain the same physical shape she had when she was 13, remain a virgin, and "learn to fulfill domestic chores" as part of the marriage contract.
The police's year-long investigation concluded that the father had "behaved for many years as if his daughter was an object" and "his property".
The teenager was able to raise the alarm thanks to a game she was allowed to play offline on her phone. Using an open WiFi connection, she communicated her ordeal to a friend in Sicily who she met in a chat room.
The friend then alerted a centre monitoring domestic violence in Florence - but she only knew the girl's name, not her location.
Police were able to track down the family and rescue the girl, whose 49-year-old father, a Serbian national, has now been arrested.