Gst on under construction property

I recently paid 23L (after the GST rate came down to 5% from 12%).

I received a demand letter for GST payment for 7% (this was mutually agreed rate at the time of agreement) instead of 5% which is the rate at the time of demand and payment (invoice generation)

They said they will charge 7% for the work completed before rate change, and any new work after this will be charged 5%.

Is this a true logic? I believe the GST rate is an application on the date of demand, payment and not related to work completed before the rate change. 

Please, advise.