Unregistered Sale Agreement with Private Financier

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 (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...but person Y is not showing any evidence of such transaction between him and X, instead threatening us by visiting our property. Pls suggest next steps