It is the same whether spending on salary or amortised fee though.
Why would it be different?
If Inigo is 9m wages and Roque was 9m with amortized fee and salary combined it is the same cost on books in relation to FFP.
Roque will be more than 4m a year though. That may be his salary but his fee on top will be around 7m so in total closer to 10m for a full season. It may be they try and free up half that if he comes for only half a season though.
I think transfer fees are interpreted differently in LaLiga otherwise we would have been able to spend modesty instead of focused on free signings these past years bar the levers.
For the cost on books of likes of Gundagon, Inigo, Aguero, Eric, etc over the years there are a ton of players in the =>30m range on a 5 year contract that could have been signed, who would amortize the same or less.