Sporting director Fabio Paratici has intensified contacts over Mastantuono, an Argentine playmaker who has struggled for game time since arriving at the Bernabeu, with talks between the two clubs continuing to advance and both sides working through the finer details.
The teenager, who does not turn 19 until later this month, has opened the door to a move to Florence despite interest from several major European clubs, and Real Madrid have already given their approval to the destination. What remains to be resolved is the financial structure, with the two clubs discussing a loan featuring a buy option for Fiorentina and a matching buy-back clause for Real Madrid, a format similar to the one used for Nico Paz's move to Como.
The main sticking point is Mastantuono's wages of around €3.5m a year, a figure beyond Fiorentina's current means unless Real Madrid agree to cover at least half of it themselves.
On the defensive side, Fiorentina have set their sights on Nice centre-back Antoine Mendy, according to French outlet FootMercato.
Nice have signalled they are open to a sale as the club looks to raise funds, a need sharpened by owner Jim Ratcliffe's decision to sell the club. Fiorentina face competition from Udinese in Italy for Mendy's signature, though Crystal Palace remain the clear favourites in the race for the defender at this stage.
Away from transfer business, Fiorentina are set to retire the number 10 shirt for the coming season in honour of club great Giancarlo Antognoni, a gesture confirmed by club president Giuseppe B. Commisso in a message shared through Fiorentina's official channels ahead of his upcoming visit to Florence for the club's 100th anniversary celebrations on 29 August.
The Mastantuono pursuit represents the clearest sign yet of Fiorentina's ambition this summer, with the Viola having already made several eye-catching additions to their squad and looking to add what would be considered a marquee final piece before the window closes.
Real Madrid's willingness to help fund the loan reflects the value the Spanish club still places on the Argentine's long-term development, even as they accept he needs regular football elsewhere in the short term.
Should the deal go through, it would represent one of the most significant loan arrivals in Fiorentina's recent history, giving Paratici's rebuilt squad an injection of the kind of talent capable of playing at the very top level once his adaptation to Serie A is complete.