Herrera, 24, has already been to Manchester to agree a deal with the club but as has been the case in previous transfers such as Fernando Llorente and Javi Martinez, they are making it difficult for clubs to complete the deals.
United will have been well aware of what Athletic Club were going to do and may even have expected this hurdle to arise because the tax laws in Spain are very complicated.
One reason Athletic Club may have rejected the bid is to avoid paying Real Zaragoza a percentage payment for training Herrera earlier in his career.
Ultimately, this situation is driven by politics. Athletic Club want to maintain the front they do not want Herrera to leave and by forcing the player to ensure the move goes through, it helps the club keep face because they didn’t facilitate any move.
This may be a hurdle but with a deal already agreed with Herrera and everything else in place, it ultimately shouldn’t stop the deal.