Proprietary Recruitment company - client is saying that the candidate is not associated with them

I own a Proprietary Recruitment company and one of my clients is denying to pay the invoice for the staff which is working with them since June 2021. The client is saying that the candidate is not associated with them, however, it’s not true. My client owns a coffee shop and is making all his employees work for 12hours.
I have sent 3 reminders for Invoice clearance, what further actions can I take? Please guide.