There is nothing stopping someone from creating invoices/payments with funds that do not exist. If a project is used to fund an invoice or journal entry, a warning that the project is NSF would help having to verify the project has the available funds. Possible option to override as well, so that users without proper security rights couldn't create a transaction that would be NSF. It would be even more beneficial if the system would tell you the current balance/budget for the project as you are creating the invoice/journal entry.
This is something that we really need! We don't budget for our projects because it's all based on revenues that come in. I recently had an invoice go through and make my project negative because there were insufficient funds.
i set up a similar idea in the Idea Bank to prevent A/P checks from being written to a project with a $0 balance OR give a warning before saving an A/P check that the project is $0 or will have a negative balance if the check is saved.
You can set the business rule to check against your project level budget and give you a warning. Wouldn't this solve your issue?