Dear Madhav,
Open Value in LoC should get updated when the payment has been recieved from Payer
As you explained above regarding the payment, payment will be updated in the Credit Management when payer is paid the billed amount but in the LoC (Payment Guarantee Procedure) customer will open LoC for particular amount that will be recorded in the SAP SD system for controlling and monitoring LoC amount.
Best Regards,
MH