The Black Cats have agreed a fee for the 21-year-old with Buenos Aires outfit Velez Sarsfield, but are yet to finalise the deal, and it emerged in South American media 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.
"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.
"It's an exceptional case because generally when there appears an offer from Europe, the player is the first person who wants to go.
"If the player isn't transferred, we have to sit down with the board to see what the next step will be.”