Credt Memo Posting Routine

Does any know if there is a bug with the posting of credit memo’s for stock? I have a normal posting of

CR Stock

DR Purchase

CR DCA

DR Vendor,

However I have a further posting of

DR Stock

CR Purchase

This final posting is for a different amount, I think it is somehow linked to the GRNI account.

A bit obscure I know, but this is the second time I have come across this error. This is on Nav2009 SQL.

Any help would be most appreciated

Andrew