Offset liability in gstr 3b

I had submitted GSTR 3B for July but did not file it. I had submitted it on 28th August with Rs. 300 late filing fees for CGST and Rs. 300 for SGST (Rs. 100 per day) before 1st September when late filing fees was waived off. But at that time, I had not paid the late filing fees, but only entered it in the return. Now, I couldn't offset the liability in order to file the return for July, as without filing GSTR 3B for July, I cannot file the return for August also. I paid Rs. 600 ny creating a challan yesterday. But still, I can't offset the liability. It shows "Balance insufficient in the minor head Late filing fees" although the ledger is updated with Rs. 600 balance under Penalty. Please guide.