If there are no bids you can revise the price to what's been agreed by clicking on the Revise Your Item link at the top left of the auction. After your buyer has placed a bid you can end it early (via the dropdown menu on right of item in your selling page) and select the option to sell to the highest bidder.
If there are bids you'd have to end early, cancelling the bids in the process, and relist at the agreed price.