As others have said, "If the qualifying Mac is returned, your refund may be reduced by the full amount of the Back to School Card." Under the asterisk portion of the Back to School Offer pages.
Additionally: "PROCEDURES RELATED TO PRODUCT RETURNS AND EXCHANGES: If a Qualifying Product is returned for any reason and not exchanged for another Qualifying Product required for redemption under this promotion, the Back to School Card must be returned with the Qualifying Product in order to receive a full refund.
If the Back to School Card is missing, is not returned, or has been redeemed by depositing its value into your iTunes account (even if no purchases have been made against the value of the Back to School Card), then $100 will be deducted from the exchange or refund of the Qualifying Product."