Refuse to refund site advance

Dear Sir/Madam

I have advanced for the Plot  of Rs. 20 lacs situated near yelhanka bangalore they have said it is A katha after the legal opion it was found it is  B katha we have request them to refund the amount which we have paid as advance we have only the receipt and the transation is done now they are refusing to refund the amount how do I go about on it, please suggest me the legal ground