No invoice yet for contractor

[James Woolsey]

Hi I'm a newbie. My agent recommended a contractor when I bought some apartments in chapel hill. He does great work and comes out at a moments notice. My problem is he has done several jobs for me including a water heater installation. Still I have not received a single invoice. I asked him if I could pay him, and he says he has trouble getting invoices out but he will when his girlfriend comes down to help him with the books. I've never heard of a contractor that didn't invoice a job asap and collect payment. Am I heading for trouble? Should I force the issue or just be patient? Your thoughts?