As per section 49A of CGST act, with effect from 1st February 2019, Payment of CGST/SGST should be made by utilising the credit of IGST, then credit of CGST/SGST respectively.
Here in your case, first set off IGST credit of Rs.1350 against output tax liability of Rs.1350 of CGST. And then set off the balance credit available in IGST (Rs.450) against output tax liability of Rs.1350 of SGST, and pay the balance SGST liability of Rs.900 in cash