Why won't Xero let me enter a transaction with a tracked inventory item?
Xero doesn't let you enter an invoice, credit note or receive money transaction if it means that your tracked inventory item's quantity or value falls below zero.
If you get an error message, check that you:
Are entering the correct quantity and value for your item
Have entered and approved all bills, credit notes and spend money transactions that include your item so your inventory record is up to date.