duty Drawback

My client is an exporter. In a particular previous year when they were entitled to drawback claims they had made appropriate claims and received the same. However, subsequently, a part of one of the bill was not realised within the prescribed time frame of 6 months. Now after a very long time there is a demand back of the duty drawback paid on unrealilsed bills.What is the position under law on this. In my opinion, possibly, duty drawback relating to unrealised portion of the bill may have to remitted back but not necessarily the drawback on the realised portion, though realised late. Would like to have members' views on the same.