Your case is dealt by Rule 6(3) of Service Tax Rules, 1994. As per the said rule the assessee may take the credit of such excess service tax paid by him, if the assessee:-
(a) refunded the payment or part thereof, so received for the service provided to the person from whom it was received; or
(b) has issued a credit note for the value of the service not so provided to the person to whom such an invoice has been issued.