Service tax paid on cancelled bill

Hai.........

We paid the service tax on invoices raised in the month of May and return filled  duly.

Now the above bill was cancelled in Feb 2013 and 12000/- of service tax was reversed in books of accounts.

Tax liability for the month of Feb 2013 is 17000/-(with out adjusting above 12000/-)

Que::

How much tax that we required to pay for the month of Feb 2013

and impacts in ST-3.??

and Kindly mention the references

Thanks in advance