Nirala Estate 30% demand letter against 3BHK booking .

Q: Hi Gents, i booked 3BHK in Nirala Estates 2yrs before and gave 10per booking amount that time .Now builder sent me a demand letter of 30per payment + interest @18%/annum for the complete period of 1yr 6months when noida extension was not bankable .Even though as i came to know that builders are increasing the floors in planned buildings and there are some other issues as well which wont allow the builder to ramp up the project .This demand letter also have a clause that in condition of non payment builder will cancel the booking so i am totally in dark what should i do in this condition ,please advice experts !!!!