For a business consultant or an entrepreneur refusing additional work is as important as maintaining focus on your current tasks. Jumping at every consulting of freelancing opportunity and becoming over flooded with tasks is out of the questions if you intend to keep a certain level of quality of work.
I am going to propose you to think of changing the behavior of just saying ‘no’ with a slightly different approach. It might be the case that the other party has strong reasons why they would want to hire you and better offer them the possibility to pay more for the service you are offering and begin work right away or come at a later time when you might have more free time and accept their job at the standard rate. Think this makes sense especially when such a job would fall nicely in the 80/20 principle of 80% of the benefits coming from 20% of the efforts.
It is not beneficial to refuse a great business opportunity unless you have very good reasons to do so. Better let your clients decide if having you as a consultant is important enough for them to pay a premium.