Unregistered Sale Agreement

Dear Sir / Madam My Father was one of Business Partners in Construction & Builders in Chennai. For development of Business, we had to pledge 2 properties (1 Property of Main partner (X), 1 Property of Father (Z)) with Private Financier (Person Y) in 2014 and and sale agreement was not registered till date. Now Person Y claims that Person X has paid half the amount and taken his document and demanding us to pay the remaining amount to release our document. The Transaction between X & Y was kept confidential and we were known about this, only in 2016. we asked so many questions to Person Y, as Person X has cheated us in the Business (Legal Suit against him is still pending in Madras High Court) 1. why didnt Person Y did not inform Person Z, when Person X paid and taken back his document 2. How much amount Person X paid to Person Y 3. was there valuation done of property...but person Y is not showing any evidence of such transaction between him and X, instead threatening us by visiting our property. Luckily i have taken up this matter to nearest police station and Person Y agreed not to disturb us at our premises and will take up this matter legally....there was no response from him to our 1st Legal Notice and Now, we are in the process of filing Caveat...Please suggest us with next steps that we should follow...