The Black Cats have agreed a fee with Velez Sarsfield for the 21-year-old right-back, but are yet to finalise the deal, and it emerged in South America on Monday that personal terms could be the problem.
Velez vice-president Julio Baldomar told TyC Sports: "The player and his agent say that the offer from Sunderland isn't sufficient - not the transfer fee, because the player will take 15 per cent, which is enough, but what they've offered him. They say that isn't it what they want."
He added: "It's the first time where we've arranged a transfer to a European club and after the player tells us that he wants them to offer more."
Whether or not the interested parties can negotiate a way around the sticking point remains to be seen, but Baldomar admits a failure to do so could cause his club significant problems.
He said: "If the player isn't transferred, we have to sit down with the board to see what the next step will be.
"It's an exceptional case because generally when there appears an offer from Europe, the players is the first person who wants to go.
"He isn't in agreement with his contract and we cannot do anything about that.
"We need to sit down and think calmly about what we're going to do.
"It the transfer doesn't go through it causes us delays with wages and payments. And it's not only on the footballing front that it creates problems, but also in the social part of the club, like the school for example, because the subsidies have been reduced by the city council."
- Sports & Recreation
- Velez Sarsfield