Property purchase in full cash, Is transaction and buyer are illegal ?

I sold my under construction flat in. 2015 to client for 12 lacs at half of the property value due to emergency. He given amount in full cash. Only Notary agreement was made. Later some delay he demanded to cancel his flat ask me to return his amount on market value in March 2019. And now the market value is 30 lacs. I have no money to refund then i had given 5 lacs and 5 blank name/ date cheques of 5 lacs each. But without any agreement or given in written regarding cancellation and purpose of cheques. Now some more delay he his threatening and harrassing me severely that he will deposit all cheques and filled case against me and will do FIR. EXCEPT of Flat sale agreement he not have any payment record. He had given me in full cash payment. And as per cheque. I have not written name and date and not have any agreement or notes while delivery of cheques. And client not have any purpose reason of cheque. So what can i do now. Can i stop the cheque declaring reason of cheques lost or