Input ITC Claimed but payment not paid to suppliers

You can issue credit note against that Tax invoices for last 6 mnths to reduce your taxliabilty, but in that you can just save your GST amount you are not gonna receive amount of goods which you have supplied to that customer.
Further there is a law if some one is not paying the amount related to Tax Invoice within a six month than he should reverse all input of that invoices ..... but there is no actionable law against it, means if anyone is not reversing their input the supplier can't take any action about this.

so, I think you should talk with your customer to pay the amount.

Dharmender Bhai, thank u so much for your kind assistance. So, shall we issue credit Note for Oct to Mar only ? as well we didn't raise invoice for Jan to Mar.

issuing credit note means you are reducing your claim of value of material/service too.

Pl reconsider.