The Argentinian international has been linked with a move to the Black Cats throughout the summer.
However, it appears that a breakthrough between the two clubs has been made after Calello confirmed they had "reached an agreement".
"We talked about the payment conditions and the values, we reached an agreement quickly," Calello told Rock & Pop.
"Now Gino Peruzzi's representatives and the English club need to reach an agreement.
"When this kind of proposal comes, it's hard to say no. If we turn them down, we might never receive this kind of proposal again."
The 21-year-old has made four appearances for his national side after making his debut in 2012.