Email 1: informal (to a friend)
Hi Jess,
Thanks so much for inviting me to your birthday dinner! I'm really sorry, but I can't come on Saturday. My manager has asked me to work an extra shift, and I couldn't say no.
Can we celebrate next week instead? I'd love to take you out for lunch, maybe at that new Thai place near the station. Let me know which day suits you.
Have a brilliant time on Saturday!
Love,
Maria