I'm afraid it's far too late to remove that feedback.
However, you did get off lightly, as the buyer left it on a Neutral and this does not affect your feedback % score, and they also didn't know feedback rules, because the negative you left them if they called CS it would have been removed, and you would have had a policy account violation strike.
There's a reason there's no ' red' negative to leave for a buyer, as it's against the rules.
@ajf-2002