MIRO - Credit memo posted before invoice

Hello,

I have a problem with MIRO.

User is able to introduce credit memo to purchase order even if there was no invoice yet.

The strange thing is that when he introduce PO number in MIRO (credit memo) system issue a message __M8080 Reversal quantity greater than quantity invoiced to date__ and that is ok but after user will click on multi account assignment and get the same mesaage "Reverse qty greater than qty credited so far Message no. M8476" but after he close the pop up with account assignemnt and back to main screen there is no message M8080 any more and user is abl;e to post credit memo with negative quantity what finally issue ABAP error DYNPRO_FIELD_CONVERSION

Screen name.............. "SAPLMR1M"

Screen number............ 6310

Screen field............. "DRSEG-REMNG"

Error text............... "FX015: Sign lost.".

When I would like to display invoice (credit memo) in MIR4 or from ME23n history.

But the main question is why system clears message M8080 after clik on multi account assignement.?

Thanks in advance for your help.....

Edited by: PG on Sep 2, 2011 10:19 AM