Unless you work for the mafia or a drug lord, and illegal defines your trade, you have all right to say no to requests of the kind. Sometimes, a client may have asked your boss to do something illegal, such as faking a receipt or intentionally escalating the cost of your product, and their request ends up on your desk. Even though there is a business mantra that insists “the customer is always right”, anyone violating the law is not right. The pressure may be tight as your boss hopes to make a few extra bucks from the “deal”, but you have to say no, however difficult the situation may be. Keep in mind that if the authorities catch up with you, an obligation to follow your boss’ orders, if you knew what you were doing was wrong, will not protect you from criminal charges.
A good way to say no in such a situation would be to remind your boss of the company’s ethics code, what the company stands for and where you draw your lines. Should your boss insist, report the matter to your human resource department or use as the ethical hotline if your organization has one, or a governmental agency such the Department of Labor.