Hello Golden, if I haven't got the wrong end of the stick, it sounds like you send the buyer money, rather than doing an actual refund.
To do a partial refund you need to go into your PayPal account and find the original transaction and click on 'details' in the column next to the record of the transaction... when that page opens you will see a 'refund' button at the bottom of the page. You must issue the refund from there, so the buyer won't be charged any fees for accepting the money.
If you just send the buyer money via PayPal, (not linked to original transaction) the buyer would be charged the usual transaction fees.