So say Everton bought a player for £30m on a 3 year contract and also sold a player for £30m.When you buy a player the the fee is spread over the duration of the contract. When you sell a player you receive all of fee into your books.
Very simplistic but it works something like this. In their accounts it would show as:
Revenue
23/24: £30m
Cost
23/24: £10m
24/25 £10m
25/26: £10m
So the 2 transactions essentially give Everton £20m profit in this year.