There’s no law that I’m aware of that says you can’t say no. If you do though, you better have explored all the alternatives first and you better have some options for your customer.
If I came to you to buy something and you can’t build it because you don’t have capacity, or you are over booked, or you don’t have the skills, I’d rather you tell me the truth. I can deal with setbacks. I can’t deal with surprises. Better yet, if you say no but give me some alternatives, even if it’s the competition, I’ll still come back to you next time because you demonstrated something I’m looking for in a supplier that’s hard to find and it’s cherished by me. Honesty and looking out for my interests.
If I’m your boss and you tell me no you cannot do it, please tell me why. If it’s because you don’t feel like it, that won’t cut it. But if it’s because you’re booked for work, tell me why and also tell me if I really want it done, then what do I have to give up in return for getting it done. If it’s because you don’t know how to perform the task, I hope you’ll still tell me and together we’ll see if there’s a way to teach you a new skill that will benefit you.
The above assumes I’m a reasonable boss. If I’m a jerk, then you may not be able to say no but I’d tell you this as the blog writer, document it. Document the fact you were told to do it, document what you were told, and what will be late because of it so you are covered for down the road. You’ll need this because you’ll be brushing up your resume to either transfer out of the department or finding a new job. Realize too, in most cases the world won’t come to an end.
Back to the customer. The Sales Manager realistically is more concerned with "have we taken care of the customer" and as a boss I AM concerned with seeing you succeed. We fear saying “No” for a couple reasons. Business wise we seem to glorify the “Cowboy”, The person who on their own moves mountains on their own to get a job done regardless of the consequences. Another reason is human nature. We seem to think the worst is going to happen when confronted by negative events.
Another reason might be you’re like me. You’re wired to help people and it’s darn difficult to say no. I’ve learned that if I need to say no I need to explain why. I need to tell others I understand the importance of what they’re asking but I don’t have the time, skills or passions to help them. I do give them some options also.