If you funded the original Paypal payment to the seller with a debit credit card then it won't go back to your paypal balance.
Once marked as completed it will automatically go back to your bank account / card but takes up to a week to process through to the bank. (Paypal say allow 30 days but I have never waited longer than a week for mine, the last one taking 4 days).
Advice is voluntarily given. Kudos / Solution appreciated if I helped you.