Is it illegal for a sales rep to modify a contract by changing the amount collected from the customer? What law if any is broken? How do I prosecute?

I am the owner of a security alarm company. One of my sales reps has been charging customers one price for the equipment and turning in a different copy to me of that part of the 4 page contract which shows the amount collected. Example: Salesman charges customer $100 for a smoke detector and leaves customer a copy of showing he collected the $100. Then salesman turns in a different copy of the page showing the charges for equipment and lists the smoke detector as 0 (zero). The salesman then pockets the $100, and I deduct $25 for the cost of equipment from his check. Normally if he charged the customer $100 he would only be entitled to 50% of the profit, $37.50. ($100 collected -$25 cost = $75 x 50% = $37.50)