Check Stock Holdings:
In MoneyWorks Gold only, if you want to be warned of an out-of-stock condition as you enter a sales transaction, turn this option on. This may slow down data entry.
— Page 190 of the MoneyWorks User Guide
From the Document Preferences, Data Entry tab, check the checkbox for “Check stock holdings”.
When the selling quantity is more than the stock on hand, MoneyWorks will warn you -- "Out of Stock".