Usurpation of Real Rights to Property

In 2008,our cousin kept on asking my mom that he will buy a portion of our property (' daanan ng tao or kalabaw') to quote, since my mom is so soft and 'maawain' she made somewhat a receipt indicating the amount which i 6,000 pesos but instead of 'daanan' she made it 'riceland' out confusion, since that cousin of ours is so insistent. Then 3 weeks ago, the wife of our cousin went to our house with a deed of sale indicating the square meter which is 704. I told my mom not to sign because that is too big for a 6,000. Besides, my mother told us before that it was only 'daanan' not riceland, that wife came over by the house again twice but my mother did not sign. He made a statement of 'magkakagulo',we reported that to the PNP since this person is in parole for murder. We were mediated at the brgy. he was the one who said that we'll just give the 6,000 to him the following week, but he change his decision the following week, he wants to make it bigger, instead of 6t, he wants us to pay the interest. We told him, we don't owe you money besides, he even use the land for farming without our knowledge and build a house there. We are the one who is the victim here, we cannot just sell a 704 sq.meter for just 6t, in fact he was the one who computed how many sq.meter will he own, because he knows our land. His father had been tendering our land.