The easiest configuration is to export an excel file or CSV from FileMaker, then import it to Sage. If your billing/expense payout is only once a month, then you would only need to run this export/import with that much frequency. The export script might take as little as five minutes to make, and then training a sage user to do the import, a few minutes more.
This video covers a sample of the format of the excel file and an import:
If you need a higher frequency (daily, realtime) then you will need to look for an integration, which is fairly complicated to casually explain here, but their new SData 2.0 protocol supports REST actions.
We suggest you have a look at www.realisable.co.uk - IMan provides a link between any Sage product, including Sage x3 and a MySQL database, which you could then use ESS from FileMaker.
You can pretty much integrate any part of a Sage product using this as middleware to FileMaker. We do have direct access to the developers if you need further information, please private message me if so.