Urgent query in case of dutiable & exempted goods

One of our client is providing both dutiable & exempted services. During last financial year 2007-08, it has been availing credit in accordance with rule 6(3)(c) of Cenvat credit rules, 2004. Suppose during the last year it has collected Rs. 1 Crore as Service Tax and it has total credit available of Rs. 50 Lakh. As per rules it took credit of Rs. 20 lakh. Now my query is whether the remaining 30 lakhs can be carried forward and claimed in the current year or it should be written off in the last year financial statements.