Hello,
this is a check thats build in Sales & Receivable Setup. Neg. Inventory Allowed. This disallows inventory going below zero. In NAV2009 this is setup general for all items. In NAV2013 R2 this can be setup for individual items. Becarefull with negative Inventory and inventory periods. Inventory periods can't be closed if there are outgoing item ledger entries that arent applied.
Jan