The source told ESPN that River's offer, which Tyc Sports reported to include a transfer fee of close to $10 million, is "below the team's valuation" and that River will need to increase its offer in order to get a deal done. ESPN previously reported that the Blues would require a transfer fee of $15m in order to sanction the departure of Castellanos.
Castellanos is in Costa Rica with NYCFC ahead of Tuesday's CONCACAF Champions League first-leg match against Santos de Guapiles.
"The ball is in River's court," the source added.
Since the conclusion of the 2021 season, one in which NYCFC claimed its first MLS Cup, Castellanos has drawn interest from across South America. Brazilian side and Club World Cup finalist Palmeiras made an offer of about $12.5m, which the MLS team rejected, although sources tell ESPN that the Verdao remain interested in the striker. Fellow Brazilian outfit Internacional has also expressed interest.
Castellanos has spoken of his dream to one day play for Marcelo Gallardo's River, and the manager recently confirmed that the NYCFC striker was among the club's targets, though there are others. River's hope is to count on Castellanos for the start of the 2022 Copa Libertadores, which begins in April.
A native of Mendoza, Argentina, Castellanos originally joined NYCFC on loan in 2018 from Uruguayan side Montevideo City Torque, a fellow member of the City Football Group -- along with Manchester City, Girona, Melbourne City and Yokohama F. Marinos, among others. The Blues made the move permanent at the conclusion of that season.
In 35 league and playoff appearances in 2021, Castellanos scored 22 goals and added eight assists while winning the Golden Boot. He also scored in the MLS Cup final triumph over the Portland Timbers. In parts of four seasons with NYCFC, he scored 40 goals with 18 assists in 99 league and playoff appearances.