Athletic Bilbao have turned down a €36m (£29m) bid from Manchester United for midfielder Ander Herrera even though the bid meets the player's release clause, as they don't want to show their supporters that they are open to selling any of their stars.

The Spanish are still bound to a Basque-only players rule, which makes it very difficult for them to replace their players even though they have a healthy balanced sheet.

With this in mind, Athletic officials have always expressed their firm intention to block any negotiations for their players - so Thursday's revelation that they have blocked United's bid is nothing new.

Manchester United tried to secure Herrera's services in an amicable way by meeting his €36m release clause in full, however the bid has been turned down as a show of strength.

Furthermore, according to the Spanish regulations, the money for the buy-out clause has to be paid to the Spanish football federation by Ander Herrera himself, rather than Manchester United. The player must buy out his own contract in order to leave the club.

So should the Premier League giants channel the full amount to the Spanish Football Federation through Ander Herrera, it looks as though the deal will be concluded well before the end of the month, when Herrera's buyout clauses rises to €40m.

However, there are two key points for United to worry about - and here's where it gets complicated.

Furthermore, Athletic can still ask the Spanish Revenue authorities to study the case – and they could force United to pay an extra charge in terms of personal income tax (PIT) in an attempt to scupper the bid. The club's trenchant stance has brought problems before; in the summer of 2012, for example, Bayern Munich's signing of Javi Martinez was delayed for two months due to complications over tax liabilities.

Secondly, Real Zaragoza - Herrera's former side, who nurtured him through the youth ranks - are demanding €1.4m from the sale. Had Athletic accepted United's 'friendly' bid the Spanish side would have to pay that amount. However, by meeting the buyout clause, United have made what amounts to a hostile offer, and are thus obliged to pay the Zaragoza fee.

IBTimes UK has spoken to lawyers in Spain, who tell us that United can avoid the extra payment to the Spanish revenue should they complete the move before the end of the month. However, in that case Herrera would have to live at least half the year in England and have to declare his earnings to the English authorities, rather than the Spanish.

United are almost certain to have to pay the money demanded by Zaragoza, meaning the eventual deal will cost €37.4m. However we understand that Herrera is likely to reduce his salary to mitigate this extra cost.

One thing's for sure, Bilbao have once again proved themselves among the toughest negotiators in football.

They forced Bayern to pay €40m for Martinez in 2012 following the tax wrangle, while they refused to allow Juventus to agree a cut-priced deal for Fernando Llorente, forcing the striker to run down his contract to secure a move to Turin.

Now the Basques are repeating the same formula with the negotiations over Herrera's move to Old Trafford and United will have to tread very carefully indeed.