Solomona was the stand-out player for Cas last season, scoring 42 tries including a record 40 in Super League, as Tigers finished sixth in the Super League standings but failed to make the play-offs after the Super 8s.
The 23-year-old has yet to show up for pre-season training and was seen socialising with the Sale players despite Castleford chairman Steve Gill saying back in October that he was not for sale.
Diamond has now made it clear that his club are keen on the New Zealander and sees no reason why he should not be allowed to switch to the 15-man code.
"Denny Solomona has resigned from playing rugby league and us, and many other clubs, are interested in signing him," he told BBC Sport. "He's got a friendship with [Sale centre] Sam Tuitupou and he's been to have a look around and at this moment that is where we're at with it.
"There are no governing body registrations between the sports. It is just like someone coming from another sport to rugby union, whether it be football or basketball and I don't understand what the furore is about."