First check System Option # 80.

Use menu path: System >  Preferences > System Options tab > Scroll down to System Option #80 - "Prevent oversell of items". It must be checked for this option to be available in Inventory File Maintenance.

 

Next set up Inventory File Maintenance.

1. Use menu path: File > Inventory Maintenance.

2. Find the desired inventory item.  

3.Click to check the box for Allow Oversell.  
4.Click the Apply button now the oversell restriction is bypassed for that item.
5.Click OK when done with item updates.