If you choose to reject the dispute, the following 3 things can happen:
The issuer rules in your favor: the transaction is amount is again charged to the customer’s account and the funds are returned to your account.
The issuer rules in favor of the cardholder. The issuer’s case stands unless you choose to pursue arbitration.
You win, but the issuer files a second chargeback: the issuer has the right to file a second chargeback (called a per-arbitration chargeback by Visa) for the same transaction.