When is a deal between two clubs not a straight deal? When it involves a third party.
Spurs found out Joao Moutinho's economic interests were not just owned by Porto but a third party investor who wanted to be paid his share of the transfer fee. It's all quite complex and the Guardian has the goods. The English league bans third party ownership. Which meant for this deal to go through Porto had to pay off the third party at their end before Moutinho could move. As a result they went back on the price they had agreed to previously and raised it further just an hour before deadline which Spurs did not agree to.
Interestingly, one of Andres Villas Boas big coups while Porto manager was landing Moutinho from Sporting Lisbon in the summer of 2010 which included a third party payoff and buyback. Moutinho became an instrumental part of that unbeatable Porto team that swept aside the league.
AVB probably had working knowledge of these financial transactions. Which makes you wonder did he did not press harder knowing these were the potential stumbling blocks? The Spurs manager called Moutinho's arrival impossible stating the club could not afford him.