I agree with @Klaus broadly speaking. Fuck the bean counters. Football is what matters for a football organization, none of this trumped up finance bullshit.
However, it seems to me that Sporting are a real pack of cunts. We agree something, then they change it, which means we have to go back and run the numbers. We agree to that, and they change it again, saying now we're responsible for solidarity payments or whatever. Then again, with the payment structure for the add-ons. Who knows what else we've agreed to and they've gone and changed it on us. You can't just say "fuck it, sure" in a negotiation. You have to do due diligence and make sure that it all works and that the new wording, numbers, etc. are all solid, even if you agree in principle.
You also don't know what kind of timeline they are forcing on us. I've been in situations where someone says "agree right here right now or we'll have to wait until next week", when they know full-well it's impossible to make that kind of off-hand commitment with millions on the line. It's a stalling tactic that might benefit them in some other way, while making Arsenal look bad because "it's just a few million".
It seems to me that Sporting have played this really, really poorly, and now they're bitter and upset about it. They feel they could've gotten more money for their best player of all time, but they were cavalier with their promises and did not anticipate that the player would essentially unilaterally eliminate all competition for his contract. They're probably doing this on purpose, knowing that Arsenal will be the ones who look bad for penny-pinching, purely out of spite.