Government contractor gross value

One of My Client is Government Contractor at the March end he received a Cheque of Rs.10,65,286/- but the part bill of the cheque and the Total Contract Value is  Rs.12,68,786/-  I want to pay tax 12% for which amount if i show the cheque amount alone and pay tax while filing IT i will show the contract amount as Rs.1268786/- kindly tell foe which amount i should pay the GST?? From department i even dont know the total value at the time of receiving the cheque