Is it illegal for an independent contractor sales rep to modify a contract by changing the amount collected from the customer? Can I hold his pay?

I am the owner of a security alarm company. One of my independent contractor 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. He pays me for the cost of equipment by deducting it from his pay requests, but keeps 100% of the profit, instead of the 50% of the profit we had previously agreed to. After I discovered this I terminated our agreement and have not paid him for his last 5 sales. Can I keep his pay? Exp: Salesman(SM) charges customer $100 for a smoke detector and leaves customer a copy of showing he collected the $100. Then SM turns in a different copy of the page showing the charges for equipment and lists the smoke detector as 0 (zero). The SM then keeps the $100, deducts $25 for the cost of equipment from pay request. 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