you can refund the entire amount. Click the transaction page. There you find a refund link to click, which puts you on another page where:

"Issue Refund ..... You can refund all or part of your buyer’s payment for 60 days after the buyer sent the original payment. When you refund a payment for goods or services, we contribute the variable portion of the original transaction fee to the refund, and we keep the fixed-fee portion of that fee from your account. For more details, refer to the page How do PayPal refunds work? or the section that discusses fees in your User Agreement. .... To issue a refund, enter the amount in the Refund Amount field and click Continue. "

Robert