Issue When cancel the SD-Billing Document

Hi,

When i cancel the billing document It should debit the Sales account with posting key 40 & credit the Customer Account with posting key 12 (Reverse invoice).

But in my case it is debiting the Sales account with Posting key 40 and Credit the customer with posting key 11(Credit memo) which is wrong posting key in this case.

Kindly advice me how to correct this wrong posting key issue and how can i fix this with correct posting key 12 instead of 11?

Kindly help me and give some inputs.

Thanks

Supriya