Hi all ;
You can get details for this problem with BIG PROBLEM / Negative stock altough not allowed in customizing .
Also please look at documents :
Negative stock analysis ( Material Management Inventory [MM-IM])
http://scn.sap.com/community/erp/logistics-mm/blog/2013/09/09/tips-tricks-about-mm-im#comment-408211
Regards
M.Ozgur Unal