"But the NRL will not consider any contract for registration until he has completed his contract with other sports.
"The match suspension he needs to serve will only begin after he has completed his playing commitments elsewhere."
Toulon has long been a sanctuary for troubled NRL stars. Sonny Bill Williams walked out on his Canterbury Bulldogs contract to link with them and Semi Radradra will join the Top 14 club later this year.
The NRL are yet to register Barba's contract with Cronulla, meaning Barba is free to link with Toulon. But if he decides to fulfil the newly signed rugby deal, it would make a NRL return almost impossible.
French rugby's season does not finish until mid-2017 and then Barba would begin his 12-week suspension, once he links back up with the Sharks. This would mean the fullback would miss almost the rest of the season.
As a result of the NRL putting their foot down regarding his suspension, Barba has said he may never play rugby league again.
"If it comes to that, I might never play the game again," Barba said. "It's taken them three months and nothing's been sorted out.
"The NRL haven't given me too much. I've got to look after my kids and my family - they're my first option.
"Nothing's been final yet. At the moment, I'm just listening to the people that take care of me and I go from there."
The NRL concede that they have made concessions for Barba. They recently allowed him to begin training with the Sharks, although with conditions.
He can only do individual training and must not participate in team activities, but he can work with the club in their community programs.
Approval was given to help aid Barba's rehabilitation, allowing him to access the club's welfare and education support services. It's the second time he's failed a drug test in two years.
Should Barba link with Toulon and, as a result, shun his return to the Sharks, the NRL have said it could potentially void the agreement made regarding his return and registration being approved.
"If that commitment [to the Sharks] has changed, then it will be taken into account when we consider whether his contract should be registered," Greenberg said.
"I would hope that everyone involved in this matter is looking at how we look after Ben's welfare first and foremost - and football matters should be secondary."