Yes, you can do a refund that way. Login to PayPal, click on the Transaction in the list of recent transactions, scroll down below the contact info (right hand side on desktop) and click "Refund this payment"
It refunds everything including fees. Very simple and quick to do.. If the transaction isn't listed under the recent transactions when you first login then click "Activity" at the top and find it from there.
Edit - sorry, I thought you were trying to send a full/partial refund. It sounds like you are trying to send new money to someone who already sent you money before.