How do I bill fixed projects after reaching contract amount?

If you have a fixed project for which the amount billed has reached the contract amount, you can go ahead and bill it beyond the contract amount without triggering an evaluation rule against it. This can be achieved by marking the additional time entries as Extra to indicate they are over and above the contract amount. Otherwise, the entries are flagged as non-billable, by default. Check CORE Help Center for details on the Extra flag.