Another scenario is that Cub G unexpectidly buys player h and player t from Club M
then also puts in a low bid for player z from Club r. Club R thinks "oh they've been splashing the cash recently, maybe they have a new backer with lots of money - they can pay full whack for player z"
....And so on
The only time it can't happen like that is if the club is a PLC as these dealings have to be made public to the shareholders
It tends to happen a lot on deadline day, but the value will be disclosed later. I think the selling club just wants to keep quiet how much they have just received, in case the price of a player they are buying suddenly jumps up as a result.
Say Colchester are trying to sign Cullip as a replacement for Halford?
They could be about to sign him at an agreed £100k, only for the story to break that they have just sold Halford for £2.5m, and Forest think "Hullo, they have got loadsamoney" and bump up the price of Cullip to £250k at the very last minute.
Nah, much better for Colchester to be able to say undisclosed and if Forest ask you can just refuse to tell them or lie. "Nah, we're not getting much cash, it's all tied up in add-ons. Anyway, 100 grand y'say!"