Unable to reverse the GR against process order on previous date.

Dear PP Gurus,

We have a cutomised T code for confirmation reversal.Where we can post the revrsal on ony posting date. The issue is as below. The user confirmed some qty let say 1000 KG in previous month. A perticular order was confirmed incorrectly on 30.09.2012 for 100 KG. The user revrsed the FG posting (102) on 01.10.2012 by mistake.

Because of this entry the month end stock was not matching.Since we had to close the month we wanted to cancel this reversal and post it on 30.09.2012.

-We cancelled the document thru MBST it gave 101 movement on 01.10.2012. then tried posting 102 to on 30.09.2012 but system was giving deficit of stock.Reversed this entry thrugh MBST.

-While doing MBST gave the posting date as 30.09.2012.then posted the 102 on 30.09.2012.System allowed.But there was 101 mvt for 100KG and 102 Mvt for 100 KG on 30.09.2012 so this didnt solve the purpose.

-We created a seperate Prc order for 100 KG posted the GRN on 01.10.2012 and then did BMST for the wrong reversal,after.that reversed the wrong confirmation on 30.09.2012. Sytem allowed.

Here i am not understanding why system didnot allow in the first case and allowed after we did GRN in this month. The stock was present on the posting date 30.09.2012 in all the cases.

Please find the screenshot as the attachment for the movements and let me know concept.

Regards

Ankush