Yeah, La Liga publish those figures in September and February when they have a total overview of what clubs have done but clubs could not operate during summer (or January) transfer window if they wouldn't know how much they can spend. They might not always have the exact numbers because La Liga can reject some of the expected incomes clubs are counting on (like they did with our VIP box sales last year) but they should know approximately where they stand and are also constantly communicating with La Liga.
Football is dynamic, not just on the pitch but also it's economy. Let's use a newly promoted club like Oviedo as an example. They will get significantly more TV money after promotion. Don't know the exact numbers but here is comparision for distribution of TV rights between La Liga and Segunda from the 23/24 season.
We are talking about around a 35m difference.
Additionaly Oviedo will probably sign new and/or improved sponsorship deals as well and earn more money from season tickets. Don't know about last season but in 23/24 they had 21,5k season tickets holders in Segunda. They have a loyal fan base, stadium capacity of Estadio Carlos Tartiere is 30k and I'm sure playing in La Liga after 24 years they will sell even more tickets even with expected increased tickets prices playing against Barca, RM and AM instead of Burgos and Cartagena.
All these revenues will drastically change their financial situation considering Oviedo's salary limit in Segunda was around 13m so it's fair they know how much they can spend on new players and build as competitive team as they can and not wait until September for La Liga's official SCL calculations when TW is already closed.