Builder has left work unfinished , after taking massive advance.

We have a plot in a farm house complex in Chhindwara, MP state. The owner of one of the plots in this complex started building his farmhouse and as we were also planning to build a small farm house in our plot, we were introduced to him. He suggested that he was willing to execute our structure along with his, as it would be beneficial for both of us for logistics, and we agreed to an estimate given by him on Whatsapp, although no formal legal agreement was made. Out of an estimate of Rs 965,000/- for the entire structure, he insisted on series of advance totaling Rs 855,000/-, over a period of 40 days or so in March and april 2015, which was paid, by eft transfer to his account. We also have transcripts of wahatsapp conversations on this matter. By the time the last transfer of 2.5L was made, he had completed work worth approximately Rs 3.5L, and after that he started avoiding our calls and making excuses for not proceeding with the works. This state of affairs continued for about 4 mths, after which he whatsapped us that the cost of construction done has already crossed 9 L, and in fact we should pay for extra expense incurred over and above what was already paid. We got the structure valued by an authorised valuer who reported that by no stretch of imagination the cost of work done could be more than 4 L, so clearly the man is cheating us by more than 4.5L . We sent a legal notice for refund of remaining amount, but yesterday he responded with a reverse notice making several bogus claims of over expenditure and that we are harassing him. Pl advise.