Sounds like you were reasonable. Some ideas have popped into my head as to how to avoid such a situation though.

1.) Send a confirmation e-mail that goes over the request one more time, including the wording he wanted. And to avoid any problems with parties tampering with the document you could get the checksum of the document and include that checksum in the documentation.

I believe the checksum will differ if someone changes the wording around or tries to add some more wording.

From the sounds of it this guy/gal didn't keep a chat log themself and therefore wasn't able to confirm it was their bad. If you send them the confirmation e-mail and they respond it was ok then I think you both have sufficient documentation and your not relying on whether or not he has a chat log enabled. Remember to tell them to save a copy of the e-mail though because its kind of a contract even though your not having him sign it.

2.) Is there anyway that you could give them the psd or whatever format the banner is in so they can edit the text themselves?

I have created a couple banners a while ago but it was never for business so I don't know how you normally do it.

Anyway, there's my 2 cents. Which isn't much with inflation. :\