Getting sued by Department of Labor

Location: New Jersey

I am getting sued by department of labor. I own a farm and an employee of mine left about a year ago, without saying anything. He’s been working at my farm since 2000 - 2016, went back to Mexico and then came back from 2020-2024. During his time in Mexico, I had lent him some money because he tried starting his own farm down there but couldn’t get it to be successful. He asked to come back to the US to work on the farm again and asked if I could pay $30K to get him back here. Since we had a strong relationship, I decided to do it for him. When he came back, I found out he had developed some type of cancer so I decided to let him work on less physical jobs, watering the green house or starting the pump in the fields. I let him work when he could and if he was feeling unwell that day, he could rest and not work. I would pay him the same time I payed my other employees and have paid him in front of my other employees as well. However one day, an employee of mine saw him leave my office and walked away off the farm. He didn’t think too much about it and just let me know what he saw. I tried to look for him to question why he was in my office but couldn’t find him. The end of the week was pay day and when I went to pay my employees, I noticed that $10K was missing from my office. Then weeks pass and I don’t see the employee at all and then I started to think that he stole my money and left. I filed a police report to find him but it did not work and the police haven’t found him. Fast forward, this past week I received a letter from Department of Labor saying that he is suing me for 3 years worth of pay. The problem I’m at right now is that I do all cash payments so there is no written slips or documentation of me paying him. I do have other employees who could testify that I paid him and have paid him in front of others. What other ways can I prove that I paid him? I’m not sure what I can do.