Feedback can not usually be withdrawn once it has been left for those involved in the transaction.  A seller and a buyer can leave follow up comments to their feedback.

If you wish to leave a follow up comment if you click on your feedback page down the bottom on the left of the page are links lncluding a follow up to feedback left - that will then take you comments you have left and you can select the transaction you wish to make an additional comment on.

Given the situation you have described I am not sure that leaving further comment for this buyer will help the situation at all.  It may make the buyer more hostile, and they too may add further comments to your feedback record, below the negative feedback they have already left for you.