The midfielder, 23, has spent his entire career with the Tefece but is out of contract at the end of this season.
Talks over a new deal this season and last have failed to produce an agreement and Casanova has called for a change in the laws.
"Le club have made him offers and, if he does not accept them, we cannot do anything," he said.
"President (Olivier) Sadran has done all he could to keep him, for 18 months now. I find it strange that players developed by a club can, of their own volition, leave for nothing.
"There needs to be a regulation which instigates a minimum payment to a player's formative club if he leaves."
Sissoko, who has been with Toulouse since 2005, is a reported target for Premier League outfits Newcastle and Sunderland, as well as Ligue 1 title chasers Marseille and ambitious Ligue 2 side Monaco.