Lake city - extra land with plot

Salam,

I have a 12 marla plot in lake city, ready for possession. I have paid for it in 5 years' installments but now lake city says the plot size is actually 1 kanal and i have to pay for extra 8 marla at current per marla price which is extremely high now. At the time of booking the plot, around 6 years back, they had told me that in case of excess land i will pay at the rate of booking price which was fine for me. Is this the normal practice in other societies? What is the best thing to do for me in this scenario?