You cannot amend that starting price now that you have made that counter offer.
What you can do is message that potential buyer ask them to kindly decline the offer if they do not wish to purchase , that then will allow you to revise your starting price to that offered.
Easier then to end the listing and relist as a Buy it Now for the agreed price for that buyer to buy from you. Make sure you have enough selling allowance to be able to relist your item.
This will show you your current selling limit, and there is a link to request an increase.
If it is not granted you are asking a little too soon.
http://pages.ebay.co.uk/help/sell/sellinglimits.html
