Person give me mentally pressure for exachang my mobile

Sir. I am medical student in russia bt at that time i am in india for my vecation i will exchange my htc one m9+ (rs.40000) with LG v10(rs.27000) from olx online selling site both mobile's worranty are over i was meet the buyer of my mobile 3 times he show mobile properly. Than. I was say show mobile properly than if you are intrsted to exchange than we will exchange this i am not say forcefully to. Exchange with my mobile . Than the day of exchange he called me and say at 11 pm to come nd exchange our phone I was put his sim nd memry card in my mobile nd say call in this card he says this is ok Than after a one day he say me give my phone back i says no than he says in angry voice give me my phone back or he says his friend is gengstar nd give me mentaly pressre to exchang this phone he come in my house nd started with bad word to me nd my family so what should i do sir. Please help me i am very depressd first day of deal i clearly says if you are interested than we will exchange this phone nd after exchange we will not repless any time he says yes Than after a deal he give me mentally pressure please help me sir