Credit Memo with Invoicing Rule - Grayed out Accounting under Rule Tab

Hi,

We are facing an issue while creating the credit memo with Invoicing Rule.

As per our contract with landloards, we have to adjust the Rent (qtrly, half yearly) from the receivable. To meet this business case, we create the Credit memo for Rent with Deferred Accounting. The Rent expenses should be accounted over a period of time. User has attached the Invoicing rule as "In Advance". Then system has grayed out the Accounting Column on Rules Tab.

User need to completing the Credit memo with Invoice Rule and Accounting details (3months, start and end date) . Then this credit memo need to apply against the invoices.