Athletic Club Bilbao have rejected Manchester United's €30 million bid for midfielder Ander Herrera.
Midfielder Herrera's contract, which expires in 2016, features a €36m release clause which will reportedly increase up to €40m next season.
Club president Josu Urrutia said: "We received a bid last night and we have told them we do not negotiate for our players.
"Our club is different. For us Athletic is a unique club. The players have to feel that.”
Urrutia's decision to make the bid public could, however, be construed as an invitation for United to bid the additional €6m to trigger the release fee.
Herrera, 24, was superb in Athletic's vibrant 5-3 aggregate Europa League win over United in 2011-12.
The former Spain Under-21 international was also a teammate of David de Gea at the 2011 European Championship Spain so convincingly won, scoring two goals in the tournament.
Critically, United's approach re-emphasises their determination to sign two central midfielders before the transfer window closes on Monday.
United have not bought a midfielder since Owen Hargreaves joined from Bayern Munich in 2007 and remain heavily dependent on Michael Carrick.
David Moyes is yet to reinforce his squad this summer, having had two joint bids rejected by former club Everton for Marouane Fellaini and Leighton Baines as well as two offers rebuffed by Barcelona for Cesc Fàbregas.