SAFPU national organiser Cappy Matutoane has revealed former Ezenkosi player Thulani Lubisi claims Sono had assaulted him after demanding his salary.
Lubisi has seen limited playing time at Cosmos and had looked to secure a move to Vardos FC, but a loan deal failed to materialise, keeping the unsettled player at the Makhulong Stadium.
He then claimed the club had stopped paying his salary in December last year, forcing Lubisi to take action.
"I went to their offices in Turfontein and found him (Sono) there. I asked him why they had stopped paying my salary," he told Sunday World.
"He got angry and said I disrespected him. He then slapped me on the face."
However, Sono has firmly denied the accusations, saying: "I don't know who Lubisi is. I never dealt with such a player.
"If he says I assaulted him, why didn't he open a case with the police? He is lying because my offices are not in Turfontein but in Rivonia. If SAFPU are taking the matter to court, let them do that."
Matutoane revealed: "Unfortunately (SAFPU) can't attend to the assault issue as it doesn't fall within our jurisdiction. We deal with labour issues.
"Another problem is that Cosmos didn't register him with the PSL. That is why the PSL was unable to help us with his case. We have instructed our attorneys to take the matter to the labour court."