But do we have any reason to believe Fabregas wants to leave Barcelona and/or that Barcelona are looking to sell Fabregas?
And do we know whether Arsenal's "right of refusal" clause operates only once, as you suggest, or does it operate in any bid that is tentaively "accepted" by Barcelona? I'd be very surprised if the clause in Arsenal's contract with Barcelona providing for the sale of Fabregas self-terminates after Barcelona rejects a third party's bid for Fabregas. The way I would expect it to work is that if Barcelona expresses an interest in accepting a bid for Fabregas that Arsenal would have a right to meet the offer from that third party. But then again, we're dealing with English and Spanish law so I have no idea what contract clauses are permissible there. Such a "third-party right to match" clause would likely run afoul of California law governing entertainment performance contracts -- at the very least, they are completely unheard of here.
But if the clause does operate as you suggest, and you may well be right about that, it would be a brilliant piece of business by United to effectively negate Arsenal's rights to Fabregas. I'm not sure how Arsenal's lawyers would have overlooked that possibility, but then again there's so much we don't know about what really goes on anything is possible.