DDA housing scheme payment for physically handicapped

1. I was allotted a LIG flat in Siraspur in DDA housing scheme 2014 under PHYSICAL HANDICAP category. 2. As per the brochure and Ministry of Urban Development (Delhi Division) office memorandum dated 9 0ct 2006 , the candidates allotted the dwellings under this category will be allotted flats on hire purchase basis where initially I will have to pay 25% cost and the balance amount in monthly installments for 15 years. 3. I got the demand letter recently where the cost of dwelling is mentioned as Rs 1,809,041.00( Rs Eighteen lakh nine thousand and forty one only) and i have to pay initial 25% amount Rs 5,43,071.00(Rs Five lakh forty three thousand and seventy one only). 4. However, the balance amount has to be paid in 180 monthly installments of Rs 16,319( Rs Sixteen thousand three hundred and nineteen only) each, making the total balance payment as Rs 29,37,420 (180 * 16319) and the total cost of dwelling goes to Rs 34,80,491 (Rs Thirty four lakh eighty thousand and four hundred and ninty one only) rather than Rs 18,09,041(Rs Eighteen lakh nine thousand and forty one only as mentioned in demand letter. 5.Will you please advice if DDA is correct in charging the interest on the balance payment when the government memorandum does not mention any sort of interest and the mode of payment is only convenience for physical handicap candidates.