The presence of Mbappe will not affect anything, and FFP will only be a bullshit for Madrid.
I've read UEFA's new rules on financial fair play allow clubs to suffer losses of €60 million over three years, compared to the previous allowance of €30 million. UEFA also imposed spending limits on salaries, transfers and agent fees to 70% of the club's total revenue in 2025/2026.
So the bigger the club's revenue, I think FFP is not a big problem for Real Madrid. Madrid's current financial condition is healthy, you can probably see it in
https://www.realmadrid.com/en-US/foundation/publications . Indeed, there are no latest reports for the 2023/2024 season. But there are not many rumors regarding real Madrid's bad financial news.