No. The system is unable to do partial refunds. You must refund the whole amount. To refund credit card transactions, use the Refund Transaction button:

• Go to Store > Transactions.
• Click on the transaction you wish to refund.
• In the upper left of the receipt, click the red button labeled Refund Transaction.
• Once refunded, this action cannot be undone.

Or, issue a refund via check or cash and log it in Accounting > New Transaction, and from the Transaction Type dropdown menu, select Donor Refund.