I'd think it's 20% of anything over what we bought him for. And surely the whole lot, otherwise we'd have sold him for £10m with a £90m extra payment if he plays a single game (plus the other bonuses).
They stuck at £80m. We could have accepted that (as suggested by some here) to avoid having a disgruntled player, but we managed to get an extra £35m! Indeed if Chelsea had offered £100m a couple of weeks ago, they'd probably have got him, but they managed to wait until Liverpool got involved...