Here is some paragraph from this book,

There's one word that everyone hates to hear and that word is "No!" This is especially true in a business setting – when your client needs that product in his hands by tomorrow and you say "No", prepare yourself for one unhappy customer.

The truth of the matter is that 'no' is a word that should rarely be heard in the successful business. The best businesspeople know how to work around that word so that even though the principle of 'no' may be present, the word itself is never spoken.

Unless it involves a matter of principle, saying no to a client is hardly ever a good idea. Your customers want you to be capable of meeting their every need, so when you tell them no it undermines the trust.

Rather than outright denying a client, seek to work around the issue and you will increase your chances of maintaining the relationship as well as keeping the trust.

When a client asks you to perform an impossible task or provide a product that cannot be obtained, your first instinct may very well be to say no. Don't! Instead, tell the client what you will do – for instance, you may not be able to finish Mr. X's monthly accounting before Wednesday but you should tell Mr. X that you will be working on it around the clock to bring it to completion.