GST on advance to builder before 31.03.19

dear expert,

I have paid 100000 to builder on 12.03.2019 for booking under construction property but agreement is made on 17.04.2019.

GST rate in March 2019 was 12 %
GST rate from April 2019 is 1 %

now builder is demanding gst @ 12 % on advance amount paid till 31.3.19.

can anyone explain the applicable rate.

thanks