The 5 million clause if a 35 million bid is rejected is true. It was confirmed by Sousa Cintra, the guy who negotiated Bruno's contract and former Sporting president.
One thing I've heard on a couple of footie shows here (not confirmed by Sporting if true or not) is that last time Sporting negotiated with banks their debt management, it was agreed that if Sporting sells players above 10 million, the banks get 40% of the fee to cover their debt. If true, Sporting either gets a big fee of it's not worth it for them, might as well keep the player.
The guy is good, for the prices being paid all over the place, it surprises me nobody has offered the 60 million Sporting are (reportedly) asking.