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I received a decline for an invitation for an individual to present at a conference and just can not find the right wording to respond to their email. Any suggestions would be appreciated!
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Doctor Jorge Foinini,

We are disappointed to hear that you will be unable to attend this year's Medical Conference, however we do realize that certain circumstances cannot always be avoided. I do hope your daughter recovers soon. Sir, we do appreciate you taking the time to consider our invitation and perhaps you will consider the Science of Medical Procedures Conference being held in January of next year. It has come to my attention that several respected members of the medical community here in Santa Cruz have been looking forward to attending your lectures on the early diagnosis of diabetes, so we have already decided to extend this invitation to you. Doctor, thank you so much for your timely response.

Sincerely,
Cynthia Eyzaguirre de Valdivia

This is my best guess. I have no idea what your conference is covering or the circumstance, but perhaps this will work as a guide to you. Let me know if you any further assistance.
Comments  

would have been nice if you had come... hope you enjoy your holidays