Action against Bhutanese buyer

Hi, My Name is Ishant and i am running a small industry of Water treatment equipments. I have supplied few plants to Bhutan as they approached us for our technology. Myself visited twice there to extend our sales and service support to them. Since we are into manufacturing industry hence minor issues like looks and asthetics are there sometime irrespective of any technical problem. We used to be very supportive to them always and tried to extent our full support as much we can. Recently we supplied 2 SS 304 Tanks and i do not know for what reason the owner of that company blaming us for sending wrong material whereas his own person from india has inspected the tank before despatch. Tge reason i am writing is he is threatening us for dragging us into the courts and second he used abusive language while texting me..where i was trying to resolve his issues calmly. The way he reacted to me was very unprofessional and wanted to teach him some lessons hence kindly guife me how i can put this up further. Regards. Ishant 9646133341