Can I discipline a worker for poor monthly sales numbers?

Q. I'm a manager and I want to discipline one of my employee for his monthly sales numbers. But he has a contract that says that before any discipline the company must have just cause. I don’t feel the burden of proof is on the company. For example, if I give him my numbers that show failure, and he gives me his calculations that show passing, I don’t know how to find out what got canceled out of employees sales. Do I have the right to discipline this employee who has different scores than I do? – Anonymous, Indiana