How do you handle btw Boss vs Client?

Although the slogan of “Customer is your boss” is definitely accurate, being a project manager, sometimes I face difficulty to balance the customer’s need and my company’s profit. Customers may require many feature or deeper feature on a system so that your project may be extended or even get loss.

You may suggest make a detail contract or requirement specification. Theoretically it sounds good but text can never cover all the things. When you can’t get satisfication from customers, they candemand a lot without violating the contract. But when you follow what they suggest, you will work and work again until die.

Have you face the similar problem? How do you handle it? Please share your experience in detail.