I would suggest you go straight to the FFP regulations from UEFA and read the Annex on the calculation of the break-even result. easily found online. Explains it better than I can, and I'd make a mistake as I am not an accountant. I'll have a go though!
As far as I understand if you buy a player for £10m over a 5 year contract, you could put that down as an expense for £2m in every reporting period up until the 5th year when the cost has gone through the 'books'. In the meantime if you sell the player then you take the proceeds and deduct the remaining cost, and that goes down as revenue (if it's a profit), and importantly it no longer goes through any more as an amortising cost
So I think it is more or less accurate to offset the amortising cost of player purchases that we have made this season against sales proceeds. You might think well wont we have to pay for those costs again next season, but a sale stops the amortisation cost process. If you sell at a loss then that is regarded as an expense.
I've probably got it wrong myself, but if so I am sure someone will correct my mistakes.