The Florence club were scrambling to sign a forward before the transfer window closes on Monday after Adrian Mutu was given a provisional ban by anti-doping authorities on Friday for twice testing positive for an appetite suppressant.
"I am staying at Sampdoria. I'm staying even if I can't deny that for some moments I saw myself leaving," Cassano told the club's website.
Fiorentina sporting director Pantaleo Corvino said he had tried many clubs to find a quick replacement for Mutu, including a failed bid for Villarreal's Giuseppe Rossi.
"I found myself in an emergency situation. We thought of Cassano," a dejected-looking Corvino told Sky TV before offering a ray of hope to Fiorentina fans.
"Keirrison is an attacker with Barcelona and we are negotiating. He is in Florence for his medical."
The 21-year-old Brazilian has spent the first half of the season on loan at Benfica.
Fiorentina, who had limited attacking options even before Mutu's ban, visit Bayern Munich in their Champions League last 16 first leg on February 17. They are 10th in Serie A.
The skilful Cassano was in stunning form at the start of the season when Samp were top of Serie A but was recently dropped by coach Luigi Del Neri after the side slid down the table.
Italian champions Inter Milan have shown interest in Cassano in the past but their attacking options were swelled by the arrival of Macedonia's Goran Pandev at the start of January.
Inter are instead looking for a midfielder to replace new Manchester City signing Patrick Vieira.
Valencia's Manuel Fernandes was having a medical but media reports say the loan deal for the Portuguese has stalled with Inter president Massimo Moratti telling reporters he was still talking to Lazio about midfielder Cristian Ledesma.
The Argentine had hoped to free himself from his Lazio deal, like Pandev, following the pair's contract disputes but a committee ruled against Ledesma and Inter will have to pay to secure his services.
