set of first from cash ledger in cgst and then from itc of igst

My question is same. If some credit is not availed and that credit becomes available after GSTR2. Can we set off liability in cash prior to setting off credit balance ?

First you have to use ITC for all (direct use first and cross utilisation later). If still liability exists you can pay by cash. If you have already paid excess cash you can use it in succeeding months. But every time you have to first exhaust ITC. Cross utilisation in cash ledger is not possible. 

can we pay set off liability from cash account before setting off from ITC ?

No. The system will not accept when you click " setoff liability" button