Regarding house purchase

Hi sir I had purchase a fully furnished house , this is my first property i had done all legal works and taken opinion and applied for loan , the property value has per the seller is 80 lacks but i got loan on my profile 48 lacks and on 28/9/17 registration date was given before that on 22/9/17 the seller started demanding me for money and he told me we can do the registration for the loan amount only and we had a agreement that i need to pay 30lack after the registration in 6 months of time on 22/9/17 agreement done and at the time of registration every one at the registration office were telling the land value is 30 lacks and the total cost will not cross 60 lacks , immediately i informed to the seller and he agreed for 65lacks and the agreement for 65 lack given to bank and registration done for 48 lacks , after the registration the seller need to finish the house with some small works but know he is demanding me for 15 lack as balance need to be given , i told him that i can not pay that much amout but he is demanding that if u do not payed the balance amount still that i can't take position of house and he had locked the house and not giving the key to me also , so sir Please let me know what can be done the property has registered in my name but i am not able to take position so please help me out from this problem sir Please