I own a two storey building in my name, although I did not have enough funds to undertake the whole construction so I asked my brother to pay for the construction of the first floor. But there was no written document. We also agreed that I would buy the flat from him after I can, when my kids grow up. He owns a separate house now, but he isn't willing to sell the property to me even when I am ready to pay more. What can I do about this?