Settlment of ledger A/c

I have one client, whom I had sold material of 600000 during 20-21 period.
due to covid 19, he is not able to pay the due amount and and we have not filled the some bill of him in gstr return.
Now after some negotiation he is agreed to pay 250000 in cash and want to closed the account and he want no due certificate from us.
Please tell me to properly accounting the same in our books of accounts