Subject: Re: [Subject of the Request]
Dear [Boss's Name],
Thank you for your email and for considering me for [specific task, project, or request]. After careful consideration, I believe I must decline this opportunity due to [brief explanation of your reason, if appropriate, e.g., current workload, conflicting priorities, etc.].
I appreciate your understanding, and I'm happy to discuss this further if needed. Please let me know how I can help