How to get original papers of the flat from builder

Plot in a registered society under U.P Awas Vikas has 3 builder flats which are sold. Now we all owners want original chain of papers from builder. But, he is insisting on providing a stamped document saying all the dues whether previous or of future will be borne by flat owner. Even though, flat registry has a language that says all the dues prior to registry will be borne by builder and future by owner. Now, for original papers he wants us to provide a document that says totally opposite, just to save himself. Though, he says there will be NO Dues. What should we do ?