How to get the mutation of property bought almost 20 years ago

i am running shop since 1993. i have owned it at 1998 but i didn't know at that time that his property was mortgaged, i don't have any proof else then cheque by which i had given money in favour of her wife (as at that time the property was of her wife now she is dead) the owner had never ask me for rent and neither given a notice. but now he is asking to do registry in today's rate.the electricity is in his name but paid by me & property tax is paid by him, what should i do ?