Dispute with the creditor over non documented money

We have a dispute with a creditor. He supplied material worth approx. Rs. 20lakh in 2013 on chalan basis but did not raise a bill for the same (out of books). We paid him in cash(no proof) periodically in subsequent years. Meanwhile, he was our regular supplier with whom with did majority business in white as well till 2015. Our accounts as per books were settled in 2013. As of today, we have an advance of Rs. 1.5 lakh given to him standing in our books since 2015. We stopped doing business with him since the end of 2015. Now, he has double-crossed us. He is threatening us to pay Rs. 20 Lakh + Interest on the unbilled amount. He says he has the signed 'delivery challans' & 2 emails that he sent on our email id mentioning the materials, quantity with challan numbers. He has been sending us various texts demanding that money for 6 months. We were advised not to reply to the text messages. Kindly advise on what cases can he file against us, how strong is his case & what remedies available?