Dembele’s deal at the Camp Nou expires at the end of this season but discussions to extend his contract have dragged on for months.
The 24-year-old has the option to sign a pre-contract agreement with a different club this month but Barcelona manager Xavi Hernandez wants Dembele to remain at the club.
Sissoko has revealed Barcelona have threatened to ostracize Dembele from the first team picture if he does not sign a new deal.
"It's applying pressure, and that doesn't work with people like us," Dembele's representative Sissoko told French broadcaster RMC Sport.
"Maybe it can work with agents who are cosy with Barcelona, but that's not the case, I am here to defend the interests of my player.
"We are not here to fuel the debates on social networks, but the truth must be told. Yes, we have demanding demands, but we have already shown that Ousmane's career choices are not dictated by the money, otherwise he wouldn't be here.
"If Barcelona wanted to negotiate, they could have tried to come to the table with us to discuss, except that there is no discussion, just threats of no longer playing in the side, and that's forbidden. We will assert Ousmane Dembele's rights if necessary.
"We don't know what we're going to do, nothing has been finalised, but [Barca's] hierarchy alone are losing Ousmane.
"From the start, we have shown that we want to discuss, with conditions of course, but without closing the door."
Dembele, whose Barcelona career has been blighted by injuries since joining from Borussia Dortmund in the summer of 2017, has made 11 appearances in all competitions this season, scoring one goal.
