I got your problem.So i am giving some solution for this.
No doubt you have a Invoice layout and user must invoiced each project.
so keep a flag field(in invoice table ) and set it to "0" first and while invoice set it to 1.
keep another calculation field e.g con1 and value =1.
Then make a self relationship among the invoice table (ID=ID and con1 not equal to flag)
this will bring all the record that are not invoiced.
Try it and hope it'll bring your result.
For starters, what you describe is a conditional value list. See if the following links will help you produce a list of projects for a selected client. Once you have that working, we can take a shot at further limiting the list to just the uninvoiced projects.
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
Well, that took a bit for me to wrap my head around. I finally got it to only populate the project drop down list with projects that are related to the preselected client. Thanks! Would it be doable to only include the projects that haven't been marked as invoiced? If it's a ton of work, I'll just manage with what I have. Honestly, I'm thrilled to have what I have so far!
A) Take a look at "option 1" in the forum tutorial.
If you set up a calculaiton that is blank except for non invoiced projects, your conditional value list can refer to that field to only list non-invoiced projects.
B) The other option is to add a calculation field that returns the same value as the data you enter to mark a project as "invoiced" in all records. Then include this field and your project status field with the inequality operator as a second pair of fields in the relationship used for your conditional value lists.