Builder is not providing GST rebate under PMAY as per latest rule change on 25.01.2018

I have purchased a property in Pune , i have booked property on 1st Jan 2018 with a booking amount of 2 lacs+ 12% GST, In last GST council meeting it has been revised that if buyer is eligible to get benefits of PMAY , revised rate of GST applicable would be 8% & not 12%. When i asked by builder he said they didn't get any circular about this. on 9th of Feb 2018 our agreement of sale has been done , in which builder mentioned that "Builder explained & provided all rebate for GST to me & no further rebate shall be asked", I opposed it before signing on agreement , so builder said when we get order copy for GST rebate we will cascade you same & for now you can sign the document. After agreement has done & i arranged a copy of circular from govt website & shared with my builder now he is saying , It doesn't matter to him & no rebate shall be provided & GST will be applicable as per 12% rate. 1) Shall i get benefits of revised GST rate, because of rule changes & it is special clause. 2) Can builder force me to pay 12% GST rate. 3) Can i pay my GST directly to govt. without involving Builder? 4) Is it so that i have signed on agreement , so its final that i have to GST with 12% rate. My second question is ,since we are in argument , i have not paid remaining amount though its mentioned in document that i have to pay balance amount in 15 days, my loan is in process & its taking time to get disbursed , My builder is threatening me to impose interest on agreement value, can builder impose any such interest on me ?