Let me help you out with a couple of quotes.
On add-ons:
"Let’s imagine: in 2021/22 Wilberforce Wanderers sell player X for £20m plus add ons depending on performance. Mr X does very well and so his add-on value rises dramatically. In 2023/4 the club with the player decide to cash in and sell him for £100m. It’s a good deal. Wilberforce Wanderers get a load of extra cash spread over five years and use it (including money not yet received) to pay off some of the debts and buy some new players. But then two years later the Revenue want a load of that money as taxation on the profitable sale of X. Wilberforce go back to the club they sold to, and say, “could you find any way to adjust your accounts…”
On sell-ons:
"But in football there is a big argument going on as to how the money from sell-on deals is accounted. Does it come into the tax year when the sale takes place or is it added back into the year when the contract is signed? And if they don’t have it…?"
I hope that helps.