According to Cadena Ser, Athletic sent a delegation including president Jon Uriarte and GM Jon Berasategi to Madrid to push for a full audit of Barcelona's finances amid their pursuit of the Spain international.
They allege Blaugrana have not followed the league’s 1:1 rule, which limits spending to the club’s generated revenue.
Williams had communicated to Athletic that he wants to leave to join Barcelona, but Radio Bilbao report that Athletic are ready to fight to keep him, and insist they have not reached any agreement with the winger to leave this summer, after he signed a new deal in December.
The player has a release clause of just over €60 million (£51m/$70m), but his current employers will call on La Liga to stop any transfer until Barca prove they can meet the 1:1 criteria.