Tough situations. if you delivered according to the agreed upon sale, stick to your guns, but be nice. No way to know he reported you. Restate as nicely as you can what was agreed upon, and what it would cost to do the new work. Send a custom extra offer with that.
Could sound like…" Dear x, I understand you would like more work done. However, in our initial order, you purchased x, which covered x. In order to do what you are asking, will cost x. Please accept the custom offer extra so I can begin on this new part of the project!."