Solomona, 23, is alleged to have demanded his wages were doubled before his controversial rugby code switch.
Court papers seen by the BBC claim Sale had been agitating for Solomona to move since last summer, and that they acted with the player and agent Andy Clarke.
The papers also allege Sale knew he was under contract until November 2018.
And they claim that Sale and the agent entered into a "cynical calculation" that they would be better off if the player breached his contract rather than negotiate a transfer fee.
The court papers include an email that Castleford say was sent by Sale's director of rugby Steve Diamond to the Tigers chief executive Steve Gill in which an offer of £50,000 compensation was made.
An earlier offer of £150,000 rising to £200,000 had been withdrawn.
In the email, it is claimed, Diamond writes: "…legal advice has been sourced and we are confident that when he walks away he will be free to play rugby union.
"I… do not want to get the lawyers involved, it isn't our style and it will be a distraction as well as expensive to go through the courts for the next two years.
"The club are prepared to pay £50,000 immediately and you will release Denny from his contract at the end of September after your last match.
"Hopefully you will see the sense in a quick, quiet deal."
Castleford are taking legal action against Solomona, his agent and the Sale club.
It is understood that the claim was only issued in the High Court of Justice in Leeds last month.
At the time of writing, attempts to contact Sale for comment had been unsuccessful but director of rugby Diamond has previously denied that the club, the player or the agent have done anything wrong.