Yeah it sounds more like a consequence of a loosely drafted contract than an immutable principle of the sell-on clause. No doubt they already come up with all sorts of language to make it harder for the buying club to fudge the fee associated with a subsequent outgoing transfer.
Reform in this area probably lies with all clubs agreeing for all "ownership" and transfers to be mediated by a regulator which provides contracts containing only standard clauses, but that'd open its own can of worms and be subject to the kind of anti-competition law stuff City Group has become so expert at.
