Not sure I get this, he hasn't signed the contract yet which is presumably why we're looking at him now. The only way what you're saying makes sense is if the Bellinghams are that in love with Brum they are asking us to give Birmingham a £25m fee as an apology. If we're being asked to give £25m away, you would have thought his parents would want it giving to him/them as a signing on fee rather than to Birmingham City, unless they are either already multi-millionaires or, idiots.
The tribunal fees, as far as I'm aware, are set at the cost to the club of developing the player. They don't generally reflect the players future value as its unquantifiable.
Occams razor says, and indeed what would make more sense is, that Duncan Castles is a fecking idiot who has no idea about u17 players and has made up a story with a juicy fee to act as click-bait.