Moving money from a bank to PayPal is not an immediate process. It takes 3-4 business days (that's in the U.S. - it actually takes longer in most other countries.) What makes an Instant Transfer payment "instant" is that PayPal is actually paying the seller on your behalf while waiting for reimbursement from your bank. When the seller refunds the payment, it's the "up front" money that's being refunded. Until the original money from your bank finishes clearing, it is still in transit and is not available for use.
The money will be returned to your Paypal account though, so no need to worry. It is then you can decide whether you add it back to your bank or leave it where it is for a future purchase.