Sell on clauses generally only apply to any increased amount a player is sold for. If you sell a player for £5 m with a 25% sell on clause and the player is resold for £7m, then you get £500k.
As you say the clause usually just involves any profit made (unlikely in this case lol) but rumours have suggested a set amount regardless which is why i was wondering if anyone knew of any truth in it. Suppose 'Arry is probably the best one to ask.