Rossi joined the outfit on Friday from Villarreal.
"He's signed a four-year deal with the option of another four years," said Prade.
"He'll have a 35m rescission valuation for the first two years and then that will be reduced to 30m for the final two."
It's understood that the Tuscan giants have handed Villarreal 9m for the player, but that could rise by another 3m if certain bonus clauses are met.
"We're happy that Pepito chose Florence because he had a lot of offers, it means that Fiorentina are doing well," Prade added.
"Giuseppe is part of a medium to long-term plan and his arrival does not mean that Stevan Jovetic will leave.
"We'll talk to Jovetic's agent soon and then we'll meet up again in June to discuss what is the best for everyone involved."