Property sold but harrassed by purchaser

I along with my Brother and Father were co-owners of a Flat in Vasai, Maharashtra. We have sold the flat and after all the formalities viz. Registration, Society transfer etc. were done, the Purchaser occupied the flat. Now after 2 months, I received a notice from the Purchaser's lawyers that too in my office address stating that we have not provided the Society Transfer documents and not paid a electricity bill of Rs. 9000.00. The letter states that if no rely is given by our in 15 days the lawyer will advice his client to file a criminal case u/s 406 and 420 of IPC against us. My question is- How can the lawyer be so naive to support his client's completely false allegations I mean, if we had not given the documents to the purchaser how in the World could he reside in the flat? Secondly, we have paid all electricity bills till the time we resided there and there has been no such mention in the agreement that we will need to bear the charges even after registration of the property then how can the lawyer quote section 406 and 420 against us? I haven't replied to the letter as I think that's a piece of rubbish. Please advice if there is anything for us to be worry about or do I go ahead and send a counter letter to the Purchaser for Harassment? Many thanks in advance for your kind assistance.