Have you been through the reports that are available under Sales Orders ?

There's an Outstanding Sales Order report which may give you what you want or you could export to Excel and experiment with the data there

You could always use the dispute facility but even then you still have to monitor payments and probably update a report to show the disputed flag. as a reminder My MD has argued for some sort of invoice approval system for years but Sage has never offered such a facility. Sometimes Excel is still the best way.