Ive explained this in detail a few times.
Heres how it works:
Player is signed for £18mil. No matter what the agreement on how this is paid between clubs, whether all immediate or staggered its irrelevant for FFP.
Lets say that player has signed a 5 year deal which means the £18mil ÷ 5 equates to £3.6mil a year dropping off his transfer fee and showing a decreasing 'book value'.
Now, i cant be bothered to calculate month to month but Digne extended his contract in Feb 2021. At that time he had played for the club from summer 2018 - early 2021.
Lets call it 2.5 seasons or £3.6mil × 2.5 = £9mil.
At that point he extended his contract so his book value became £9mil which is then spread over another 4 seasons (approx) which is £2.25mil a season.
If he leaves in Jan then thats £9mil - £2.25mil = £6.75mil.
So he has a book value for FFP purposes of £6.75mil.
If the club chose to sell Digne for £26.75mil then that becomes a £20mil profit based on the current FFP book value.
Again, workings are general not exact on the months / fees.
Then simply remove the boys wages from the wage bill and that also becomes an FFP saving....instantly.
Now, to sign players its different.
Take that £20mil profit weve made above. We can use that immediately as its a 'realised' FFP profit.
That means (using the same contract calculation as before) we could spend £100mil in January.
How is that possible? Over a 5 year contract with FFP we pay £20mil a year, so that totals £100mil over 5 years.
E.g we could sign 5 players for £20mil each = £100mil.
FFP year 1 = £20mil (Digne profit)
FFP year 2 = £20mil
FFP year 3 = £20mil
FFP year 4 = £20mil
FFP year 5 = £20mil
So, in theory we can use the entire £20mil this Jan to sign £100mil worth of players...but we need to pay another £80mil over another 4 years.
Is this stupid? Well, in theory no because we will have saved wages on out of contract players of Sigurdsson, Delph, Tosun and Kenny which probably covers £15-16mil a year. Player wages are probably covered by James/Digne leaving.
So we'd be in the red by £4-5mil for the next 4 seasons which is not a big deal and easily solved with a tiny sale or contract restructuring here and there.
Again, all figures are approximate etc etc