If you sponsor a function, do you

1) raise an invoice for the full usual fee and then a credit note for the sponsorship amount?

or

2) raise an invoice for the agreed expenses only?

If you agree to do it for absolutely no charge, do you just turn up on the day or put a debit/credit through the books?

I hate accounts and nothing has been straight forward today - brains beginning to get a bit frazzled!

Number 2

Thanks definitely the easiest way.

I agree too.
For a freebie, it can be listed as such in the accounts, but still get a signed booking form for this just in case 'The Man' wants proof that you weren't paid anything.