The union has demanded that Mthombeni settles all the previous owners' debts and salaries owed to players.
The businessman is believed to have purchased the club's status for R12 million and is planning to relocate it to Mpumalanga.
"We already have cases regarding Free State Stars, four of them have been already reported," Union deputy secretary-general Calvin Motloung tells KickOff.
"The owner says he does not need them in his team.
"They were turned away and told to come back on Monday so that their contracts can be discussed.
"But he made it clear their services are no longer required.
"We will find out on Monday how many more will be informed they are no longer needed.
"So we are looking into this, we are ready, we knew this is going to happen.
"For the new owner to be able to move, he has to pay all the club's debts.
"He bought the club on a buying consent, so all the debt must be paid.
"I don't know what the Mokena's told the new owner and if they told them they had debts.
"The new owner could have done his due diligence and found out if there were debts before he could even buy the club.
"So if the new owner didn't do his due diligence, then there's a problem.
"He is liable for all the debts. We will be chasing after the new owner."