Buyer invested in an under developed layout & owner delaying work

Our family had invested 4 Lakh rs on site, in an under development layout, from a person in the town. We went for deal through a known person. Seller/owner said development will be completed and site will be registered soon. Even after years he did not develop the layout. Everytime we call/meet and ask, he used to say will complete in month or so.. but never did any progress. after few years we argued that our money could have got better interest, if invested somewhere else, then he gave an updated document of the property agreement showing 7.5 Lakhs invested (additional amount as interest given). And said to wait for some more time for layout development. Even then he did not progress with layout and he died. Later the responsibility of layout was taken over by his brother. It is been 3-4 years, again the new person also continued saying will complete soon. But not making any progress, neither giving the money. It was hard earned money. Please suggest how we can get this issue solved.