GST changed to 5% from 12%-

I have purchased a flat, and my registration/agreement was done in June 2018. However, the builder had not given me possession till date (6 May 2019). The GST applicable was 12% at the time of registration. The builder will get the completion certificate in June 2019, when the GST reduced to 5%.

Now my question is,
At the time of agreement/registration, I’ve paid 70% of the amount on which 12% GST was applied. But now after 1 April, 2019 GST was reduced from 12% to 5%.
In May 2019 month flat is almost ready to deliver so at that time, Builder is demanding me the remaining 30% amount at the rate of 12% GST instead of 5% so i think he is not Passing GST benefit to me.
Is the builder correct, or is he overcharging me?

Your reply would save my 1.5lakh please help.🙏