Unable to Reverse the billing Doc

Hi SAP Guru's,

When canceling a billing doc. T/code VF11 syst. refers to LOG.

The Log states : Cancellation doc. is not the same as original billing doc.

                             : Data inconsistency during processing of he doc.

The billing doc. that am try to cancel/ reverse is not cleared to account.

Can you tell me how i should go about in overcoming this issue and canceling this doc.

Thanks in Advance!!