Solomona scored 40 tries in the 2016 season, including a record total of 40 in Super League.
Tigers chairman Steve Gill said in October that the 23-year-old Samoan was not for sale and would be returning to the club on 7 November.
BBC Sport understands Solomona has held talks with rugby union side Sale.
The Premiership club declined to comment on the story.
Castleford Tigers are to take legal action against winger Denny Solomona after he failed to return to the club for pre-season training.