Agree with XPD. Make your feedback as factual as possible so others can see what has happened.Try to construct a case to support the bad feedback with dates and times and even quotes from emails.

In your case, the trader you are dealing with doesn't sound at all reasonable and you probably can't avoid bad feedback in response. But that is another opportunity to show them up when by responding to their feedback claim with more facts so your calm, reasonable and factual response will be there for all to see. When I have responded in this way, traders have even removed their bad feedback because it made their feedback look unfair and petulant, which it was. But your trader doesn't seem the type to do this.

I can understand the desire to maintain 100% positive feedback because I have that from over about 1,500 trades. The only way I've found to keep 100% positive feedback it is to treat people politely and honourably even if you end up out of pocket. You also have to avoid risky traders who overreact, are unreasonable, are rude, who don't correct bad feedback after the issue is resolved, who don't answer questions on their auctions either accurately or at all, etc. I have removed bids from such traders and I also blacklist any I find.

The negative side of having 100% positive feedback is that some traders will think that you value your reputation more highly than the trade itself. They assume that they can deal with you with impunity because you won't want to lose such a good reputation. This makes it even more important to screen the traders you deal with.

P.S. Agree with dolson.

I should add that I have placed several neutral feedbacks and a couple of negative feedbacks which have not been reciprocated because the other traders were acknowledged it was reasonable.