Stepan Chernovetskiy (in the center), Dmytro Kucher and Viktor Postol
But a deal has not been officially reached, according to Vadim Kornilov, who manages Postol.
"No, a deal has not been reached," Kornilov told in an email.
A verbal agreement is in place between Postol's side and Golden Boy Promotions, which promotes Matthysse.
But both said are waiting for HBO to give final approval on the proposed date of the fight.
When the fight becomes official, it will take place at the Stubhub Center in Carson, Calif.
Matthysse (37-3, 34KOs) and Postol (27-0, 11KOs) will battle for the vacant WBC junior welterweight title which Danny Garcia recently vacated.
The televised co-feature could be a junior welterweight showdown between former champion Humberto Soto and unbeaten Antonio Orozco.