i wish to know, is it mandatory for reversing 10% of the invoice value or can we reverse proportiante cenvat amount involved in the exempted goods without maintaing separate books for both dutiable and exempted clearances.
Reversal of cenvat
Where separate accounts are not maintained, reversal of CENVAT credit may be done by maintaining separate books, applying a statutory fixed-percentage reversal of invoice value for exempted outputs, or reversing the proportionate credit attributable to inputs and input services for exempted outputs by following the proportionment procedure in the Cenvat Credit Rules; practical difficulties in segregating input services and consequences of reversal including interest are relevant to the choice. (AI Summary)
TaxTMI 
