Don't leave a positive and then justify by leaving a negative in the comments, what does that provide except the buyer another positive feedback, making their numbers look even better.
When I received my 2 neutrals I just added a comment to the feedback they left me, explaining my side, making the buyer holding the bag, and never left them any feedback.
When you go to a sellers and buyers feedback, do you read the positives, NO, you look at the neutral and negatives, so, if a buyer gives you a neg or neutral, why would you give the buyer a positive feedback and then try and justify why they didn't deserve it.