Yes, they will be able to retaliate with the negative and probably will. You could get slick and wait until the 89th day, as unless eBay has changed that, one can only leave feedback until the 90th day. However, this system never seemed terribly precise, so sometimes people were able to leave feedback for a week or so after the 90th day.
That said, if you are only a buyer, then a single negative is not going to mean much to you. If you also sell, then it might have a greater impact. Even then, one may not make much of a dent, unless you are brand new. However, I would leave it, because it is just and the proper thing to do. I would also report your situation to eBay using their complaint resolution center. Don't get into speculations or other salaes records. Simply report what happened to you in your situation.