Yes, The buyer can revise the negative feedback to positive if you can convince him or solve the problem he is facing.
For doing this, please visit feedback revision link present in feedback forum page.
Feedback forum page can be accessed from the customer support page. You can click on the link for revising the feedback and describe the solution reached with the buyer and send the request for the buyer to approve. once the buyer clicks on that link, he will be guided to a page where he can revise the feedback to positive, neutral or negative as he wishes.
Regards
Kailash
