Could you explain further, thanks.
It's only accounted for as book value - a player is an asset that depreciates during his contract.
Aden Flint for example - bought for £7 million on a 4 year deal - sold for £4 million (up to £6 million in clauses) one year later.
At the time of his sale, Flint's "book value" was £5.25 million, so if we did receive the £6 million - it technically gives us an additional possible spend of 750k.
If for the purposes of this example we had the £7m to spend initially.... We can now spend £7.75m.
However, fans forget that 7m minus 6m is still 1m. Gibson who's essentially bank rolling the club has still lost £1 million in order for the club to be able to spend an additional 750k on another player.
-----
Assombalonga - forgetting about the additional loses in his massive wages - cost the club ~£15 million.
His value depreciates during his contract and when we released him, his "book value" is zero, so the loss is "accounted for" and we don't lose any money/available spend in terms of FFP.
However - The initial £15 million has still been spent - we don't get the physical money back, even if it hasn't been lost in terms of FFP.
-----
It's obviously great news (financially) when we're able to sell players like Akpom - who we bought for £2.75m on a 3 year contract and sold for £10.5m.
Not sure how the additional year extension works, so we'll pretend he was sold in his final year, when he was worth a "book value" of 900k. We get the £9.6m added to our "available spend".
Gibson (or the club) also makes a profit (forgetting about wages) of £7.75 million.
-----
Some fans are suggesting... "We've sold Akpom for £10.5m so we have £10.5m in the bank. Lets spend it."
We can technically spend ~£9m of it in terms of FFP - but Gibson has personally bank rolled the club through bad financial signing after bad financial signing, for years and year and years.
Just because we technically have the money available to spend - doesn't mean we physically have the money available to spend, unless Gibson gives us it... again.
-----
The fans that are complaining that Gibson won't just write off his tens of millions of loses and give us more money, are probably the same fans who complained about the season cards going up £30.