Subject: Re: [Original subject]

Dear [Name],

Thank you for thinking of [Executive name] for [opportunity / meeting / event]. I appreciate the invitation and the context you provided.

[Executive]'s diary is fully committed in [period], and after looking at the calendar carefully I'm not able to make this work without displacing existing commitments. I'd rather decline cleanly than overstretch.

If circumstances change, or if there's a different way [Executive] can support — a short note of endorsement, an introduction to someone better placed — please do come back to me.

With thanks,
[Your name]
EA to [Executive name]