Valencia have warned Manchester United the only way Nicolas Otamendi will leave the club is if he publicly tells the club he wants to move – and they meet his €50m (£36m, $56m) release clause in full.
In addition to a midfielder, United boss Louis van Gaal is reportedly keen to add a formidable and experienced defensive presence to his squad.
Otamendi, 27, is currently away at the Copa America with current United stars Angel di Maria and Marcos Rojo as part of the Argentina squad but he remains closely linked with a move that would see him join his countrymen at Old Trafford.
The centre-half's agent has already indicated the player feels he needs to move on to the next stage in his career.
But if United have decided to make Otamendi their next defensive addition, they will have to spare no expense, says Valencia's president Amadeo Salvo.
He also insists another condition will see Valencia decide the payment structure of any deal.
"Valencia do not give in to pressure from any agent or any comments. Valencia's position will always be the same," Salvo said.
"If Otamendi wants to leave he will have to say it publicly. If he wants to go he will have to come to the club, not an agent, and pay what the club decide he is worth, which is €50 million at least, and with the payment structure that Valencia decide."
United defender Rojo has done little to quell suggestions of his compatriot joining him in Manchester, with the former Sporting Lisbon defender teasing supporters by favouriting tweets asking him whether Otamendi would join the club.
Last summer, United brought in Marcos Rojo to help fill the void left by Nemanja Vidic and Rio Ferdinand, both who left Old Trafford at the end of the 2013-14 season.
Van Gaal is keen to strengthen his options further after a season where his options were often shortened by injury and suspensions.