1) I have the agreement mentioning i have paid 15lakh to the builder and I have not signed it yet as i do not want to continue now.
2) I have all the payment receipts
Can i get the full refund?
Firstly you secure the evidences of the payments made so far including the copy of agreement (though you may not have signed it);
Issue a legal notice to the builder stating that there is no transparency in the property deal and since the amount now being charged is exorbitant as well as not in the oral agreement made at the initial stage, you are hereby cancelling the agreement and direct him to refund the entire amount without any deduction because of the breach of contract on his side.
Failing to respond to the legal notice served on him., you can drag him to consumer forum seeking recovery as well as costs towards compensation for the mental turmoil/agony you suffered due to this.