Unbalance FI document

Dear Guru,

I've encountered a very strange issue: FI document already posted but total debit amount is different with total credit amount.

My finding is: the unbalance FI document was generated from Billing. And the tax condition value in Billing differed with tax amount item in FI document.

I can not understand why SAP allowed to post FI document with balance is not Zero ? There should be the final check before posing FI document...

Has anyone encountered the same issue?

Please help to advise for further investigation!

Thanks all!