The exact way that you would do this depends on which ODBC option you have set up. ESS which is essentially a "live" link into the data back end or a system of importing and exporting to/from the data source tables.
But you have put your finger on a weak point of FileMaker finds--finding data where it's the comparison of two field values that determines whether the record should be returned by the query. This is not as easy as we would like in FileMaker.
The typical method in FileMaker is to define a calculation field such as:
Product Quantity < Reorder Quantity
With Number specified as the result type.
You can then do a find for all records with a 1 in this field.
Your ODBC data source design may require that you set up a related table with this calculation in a one to one relationship before you can get this to work.
That calculation field solution worked just fine!
Thank you very much for the answer.