Builder demanding Interest on Late payment.

I book a flat in navi Mumbai in august 2015 with token of Rs-51000 and applied for loan from one agent mean while I paid remaining 69000 of 5% down payment because I was trying to sanction loan on 5% down payment but due to agent it takes 2 months delayed to take loan sanction. In October I got sanction letter. But bank did not agreed to pay loan on 5% OCR so I paid another 5%. After loan sanction I contacted builder for registration & My registration was done in Dec2015. Before that In November Builder send me Demand letter of total 80% work completion demanding 20 lack rupees without registration. mentioning 24% late charge note below demand letter. After registration in December Bank paid him 18 Lack within 20 days of registration. I have paid him total 85% of flat cost but now he send me another demand letter of 95% completion work mentioning Rs85 Thousand interest for late payment mentioning I will not give you possession till all your payment including late charges. so suggest me what should I do. can builder demand Late charges before registration. In Registration Builder mentioned 18% interest on late payment but after registration in December & I have paid all his demand on time.