If you were incorrectly sent PayPal funds, you can return the excess funds via PayPal.
Unfortunately, PayPal doesn't allow you to issue a refund directly. Instead, you'll need to use the Send Money option. Here's how:
1) Go to the Send Money Page (in PayPal under Tools)
2) Enter the email "firstname.lastname@example.org" and click Next
3) Enter the Amount and click Continue.
4) Select "PayPal Balance" for how you want to pay and click Next
5) Confirm that everything is correct and click "Send Payment Now"