I'm quite stretched thin at the moment.
C2I'm very busy and have limited time.
Unfortunately, I'm unable to assist at this time.
C2I cannot help now.
I'd be delighted to assist, but my expertise lies elsewhere.
C2I'm willing to help, but I'm not the right person.
While I appreciate the offer, I think I can manage.
C2Thank you for the offer, but I don't need help.