Manchester United missed out on signing the Athletic Bilbao midfielder Ander Herrera not because the deal was hijacked by "imposters", but due to their failure to understand the complexities of Spanish buy-out clauses. The Guardian understands that the club were not prepared to risk testing the legal framework of the system and pulled out of the deal, later seeking to distance themselves from another unsuccessful bid by claiming the lawyers were "not acting on their behalf". Yet according to well-placed sources at the Spanish league, the three men who visited their offices were indeed working for United.

Rodrigo García Lucas, Alvaro Reig Gurrea and Guillermo Gutiérrez were photographed entering the league's offices on Monday afternoon, with less than 12 hours to go in the transfer window. The three men represent the Spanish law firm Laffer, which oversaw Bayern Munich's successful move for Javi Martínez. That deal was completed by paying the buyout clause, against the wishes of Athletic Bilbao.

It is understood the lawyers had been instructed by United to attempt to find a resolution to the transfer and, according to sources at the Spanish league, did so with the formal authorisation from Manchester United.

Herrera's agents were not directly involved in the operation and nor were Athletic Bilbao; they later confirmed to the league that they would seek to block the move, refusing to accept the transfer of money.

The lawyers did not deposit the €36m stipulated in the clause and were not able to meet the necessary requirements as far as documentation was concerned but they did hold talks with league officials. They departed over an hour later citing "bureaucratic difficulties".

They later briefed United as to the difficulties of closing a deal so late in the window; they could not guarantee that it would be completed successfully and without subsequent risks. The Martínez deal, by contrast, had been put together over more than a month.

Faced with the uncertainty and potentially extra costs involved in tax, United decided not to proceed, thereby forcing Herrera to stay in Spain, the sources state. United had told the midfielder to be prepared in the event of a deal going through. Once the deal collapsed, United claimed that the three men had nothing to do with them.

Spain's official buyout clauses are complex and United appeared not to be fully aware of the difficulties inherent in using them. In principle, it is the player, not the buying club, who is supposed to deposit the money with the Spanish football authorities.

However, Manchester United insiders again insisted on Tuesday night that the lawyers were not working directly for them and the deal collapsed at an early stage because they were not prepared to meet the €36m buyout fee.