builder charging gst even after completion of building to give posession without OC

I bought a flat in Vasai east where when i gave token and payment in july 2017 my building had 4 to 5 people staying in the building that means building was ready to move but waiting foe OC to come.
I thought it is ready to move so there will be no GST
when i payed amount in july aug building was completely ready ,and only garden work was pending
now builder is saying although i had given possession to other people u are liable to pay GST
i dont how he is claiming benefit of credit of gst when his building was already ready in Vat before july 2017

KINDLY HELP ME HOW WE CAN SAVE TO PAY BIG AMOUNT IN GST AS HE IS TAKING FROM OTHER PEOPLE ALSO ALTHOUGH BUILDING IS READY TO MOVE SINCE LAST 15 MONTHS .
PLZZ H3LP WHAT COULD BE DONE .