Is there a technique that will enable me to refresh data that's in a collection w/o resorting to using modal forms?

For example, I have a main form that displays all "Invoices" (a collection object of any number of Invoice business objects) for a certain vendor in a grid. A user can select an Invoice from the grid and open a new "Invoice form" for editing. Pretty typical stuff but here's my problem: If I use a modal Invoice form I can just pass back to the main form if changes are saved by the user so that I can refresh the list. However, if I use a non-modal form so that the user can open more than one invoice form at a time for editing I don't know when to refresh the collection since I don't know when each Invoice form is saved. Any ideas?