Property Related

Our shop situated in Meerut City (Uttar Pradesh). Around 30-40 years ago my grandfather borrowed 15000 rs. from my uncle (Phupa Ji) and name the property to him. Possession was with us. The amount was not given back by my grand father. In 1992, an agreement was signed between my grand father and uncle (Phupa Ji) and amount payable was settled as rs. 50000.00. Around 20000.00 rs. was given back time to time but fully amount was not paid. My grand father has passed away. Now my uncle (Phupa Ji) is demanding around 18 Lac rs. @18% compound interest. There is no receipt with us of rs. 20000.00 though my uncle is accepting the same and amount 18 Lac is calculated after adjusting the same. Agreement, signed in 1992 is also not with us. The possession is with us since starting the dispute. There is no legal notice send by my uncle (Phupa Ji) from 1992 to till date. I am ready to pay the balance payable amount but not @18% compound interest. Kindly suggest what is my position in the case and at what % amount should be given back. There is no % written in agreement, signed in 1992.