I see what you're saying. It's a good point which underlines the fundamental problem with FFP (or any other system that ties spending to revenue).
There's a difference between profit and cash. In the Profit & Loss account, which FFP uses, transfer fees are accounted for via amortisation. So imagine spending £250m on players one summer, which you pay £50m cash for initially, with 2 further instalments of £100m after 12 and 24 months.
You'd amortise them at £50m a year, and if you make a profit of £50m before amortisation, you'd break even and be OK for FFP purposes. You might record the same numbers in your P&L Account the following year but if you didn't have that £100m cash you needed to pay the following year, then you'd need to find the money from somewhere else or go into administration if you couldn't, as nearly happened to us in 2008. Chelsea would borrow cash from RA to fund any transfer or other spending where they had a shortfall of cash. That never goes through the P&L account, which is all FFP looks at.
In La Liga they look at a club's available future cash flow and set forward spending limits based on that, which is why Barcelona got hammered this season. Tebas may be a racist/ active fascist but that's a far more sensible system than UEFA's.
Hope that explains it a bit better.