Dear XXX（人称）,
Thank you very much for your gracious invitation and I would like to have dinner very much.But my uncle is coming back from New York and I have to pick him up at the airport at that time.So it is really a pity that I could not accept your invitation.I am awfully sorry to you.If possible,I hope that we can have dinner together next time.
Yours sincerely,XX(你的名字）