Trespass of property.

On nov of 12th 2015...i had bought a house on Rupees 11 lakhs and 25 thousand and the payments were done on cash but in such a way tht 7 lakhs were been paid before signing the house deal and 4.25 lakhs after signing the deal... Now...7 lakhs were been given on the spot but 4.25 lakhs were to be given after signing the deal so they posponed the signing of the deal for a week and after a week i signed the deal in which it was written that the money given by the buyer to the reciever is 4.25 lakhs... And then they gave me the keys of the house and was told that now the house is mine.... Basically i bought the house as an intension of investment and so was not going to shift in that house... But after 2 and a half months that buyer is saying that i have not paid the 4.25 lakhs yet ... and so they are asking me to pay it or else they will occupy my house .... Now the problem is ... i refused them to pay the amount because i hav le already paid it and i have the proof of it with their signatures of recieving the amount ... but still they are claiming that i have not paid the amount of 4.25 lakhs. Now as i have refused to pay so they have occupied my house and started living in it. So what should i do to prove them wrong and get my house back ???