1) you have no evidence of cash payment made to builder
2) no receipts are issued by builder
3)mere cash withdrawal from bank is not sufficient to prove payment made to builder
4) what is basis of additional demand of Rs 1. 80 lakhs in cash . Is it provided in agreement ?
5) pay by cheque if any such clause in agreement and ask builder to give receipts of earlier cash payments made
6) you can move consumer forum against builder to seek orders to direct builder to execute sale deed and deliver possession of flat if payments made by you are as per sale agreement