If your users are logging in with a privilege set that is not [Full Access], you can set up a record level access expression that does not allow editing unless that field has a value greater than zero.
Open up the privilege set definition in Manage | Security.
Select "Custom Privileges" in the Records Drop down
Select the table where these records are defined by clicking the table name
Select "Limited..." in the Edit drop down
Enter an expression that evaluates as True when you want users with this privilege set to be able to edit a record in this table such as:
BdFt > 0
You may want to repeat this for the Delete drop down if you also want to use the same logic to control whether or not the record may be deleted.
I don't see the "Custom Privileges" or Records Drop down.
I'm running FM Pro 13 Advanced, the choices that are available in "Manage Security" are;
It's there. Choose the option to edit the account's privilege set.