Builder asking legal heir

My mother had invested in a property with a builder . Before registration she expired . I and my father want to pull out of the arrangement . The builder has asked us to get a legal heir certificate . I had applied for the same to Tahsildar office . They had forwarded to Talathi for further processing . I wrote a application to Tahsildar not siting any specific reason for legal heir as had an impression that it is a blanket document which would be useful for various reasons . Talathi is now asking me to get a document with a reason for legal heir . Went back to the builder asking him for such a letter as they are asking me to provide legal heir . They are refusing that no such communication is required . They want to delay in returning the money also . Can you help me advise in this matter .