Selling a mortgaged Flat

Hi, I own a mortgaged flat bought from the builder directly. Later on the society formed. I have an outstanding loan on the same flat. Now I wish to sell the same flat and the buyer wll finance the purchase through HL from HSBC. HSBC is asking for the NOC to transfer the flat in the buyers name. My society is refusing to give NOC and asking for an NOC from my lending bank towards the sale of flat. PLs guide me on this.