Colchester have rejected a £150,000 bid from Charlton for midfielder George Moncur, according to Sky sources.
Sky sources understand Moncur will be allowed to leave Colchester this summer, but they are looking to secure up to £500,000 for him.
Moncur has held initial discussions with Barnsley over a move to Oakwell, although the Tykes are yet to come to an agreement with Colchester.
Sky sources also understand Peterborough, who appointed Grant McCann as their new manager on Monday, are preparing to bid for Moncur, as are Milton Keynes Dons.
The 22-year-old joined Colchester on loan from West Ham in August 2014 before making the move permanent three months later.
He scored 14 times in 51 matches during the 2015-16 season, taking his goalscoring tally for the club to 23 in 97 appearances.
Moncur was one of few bright spots in a disappointing campaign for Colchester, who will play their football in the bottom tier next season after finishing in 23rd spot in League One.