If the 350k is fixed and can’t be increased even after 60 months, this is permissible.

i want to buy a flat but i am wondering if i can go with the installment plan which is interest free..but the project is like if i buy directly from an owner who already bought that flat from a builder few years a go on installment it costs me 300k but if i buys the flat from builder it costs me 350k but it will be divided on 60 monthly installments.builder have only this option