Service tax advance

Dear Sir,

I am having the following queries, please guide me on the same.

Please find below the  Facts of the Query :

I am a service provider and registered with Service Tax Department. I had billed to my associate enterprises during the F.Y. 2010-11 So as per service tax I am liable to pay service tax on accrual basis and not on receipt basis. Accordingly I had paid the service tax to the department during the F.Y. 2010-11. But in August-2011 the Management of the Associated Enterprises find that the billing already raised is incorrect. So they raised a debit note against the same. I had already filed the return for the half year ending on 31.03.2011 and also I can not revise the return, as the 90 days has been lapsed.

And now please find below the Query :

So now what is the remedy available to me regarding the Service tax already paid on the billing to associated enterprises for which i got the debit note from the associated enterprises. Should I opt for the Refund of service tax from the department or Should I treat the same as Advance service tax and adjust my current service tax liability against the same?

Please address the query on urgent basis.

Thanks