Interest On Corpus fund with Builder

I have purchased a flat in Pune. During the purchase of the Flat we were informed by the builder that we will have to pay a corpus amount and interest earned out of the deposit will be utilized for the maintenance. In the agreement to sale it is written that "The Purchaser shall on or before being placed in possession of the Flat pay to the promoter Rs. 6,85,000 /- which shall be deposited by the Purchaser with the Promoter towards Maintenance Fund. As far as possible only the interest earned on the Maintenance Fund shall be used for the Maintenance and other expenses. In the event, the interest is not sufficient, the Promoter will be entitled to utilize the Fund for the Maintenance and expenses" Now before handing over the possession the Builder has demanded 12 month advance maintenance as by then decent amount of interest will be accumulated with the corpus, we agreed and paid the 12 months advance also. Now before possession the builder wants an undertaking from us that the corpus amount will be interest free and the builder will handover the corpus to the Federal condominium once formed. The builder is not ready to handover the possession without such undertaking. Can the builder demand such undertakings? Can the builder deny/delay possession if such undertaking is not given?