My boss is skimming money off my sales, I'm commission based, what can I do?

I had a feeling it was happening so I started keeping records of all the jobs I did and the price that the company charges for those jobs. I make a twenty one percent commission on everything I did that week. Once I started keeping records, I noticed that the very high dollar jobs he's taking several hundred dollars off the top and not giving me the commission for what the company is charging the client.

What should I do?