Hello,
Not 100% guaranteed until checking it, as for C5 I did not looked for that, but as other Dynamic do have, I am almost sure that C5 does have it too - web services that can be set exposing different parts of its data.
So one approach could be:
A)
1) Exposing the WS for the required inventory data(from the product UI)
2) Given the available wsdl from point 1, making a small application to consume the web service(a console app to be executed from a task schedule, windows service, desktop app to keep in tray, library to call from other apps, could be any of these choices depending on your preference)
3) Save required data to cvs/excel
B) Other good possible approach:
Access needed data directly from the MSSQL database(if you installed the product on an MSSQL instance that you can manage, or if some ODBC driver available). Scheduled save can be directly from MSSQL programmability, or from an outside app accessing the MSSQL database.
First choice would be most robust(independent of changes in database), second choice would be fastest(and reliable too, considering that you do not have complex data to read just ID and Qty, small chances to be changed in database structure for them on updates). Any of the two would solve your need well, if huge quantity of products I would choose directly db.
Any of the two ways would fit in given budget and doable same or next day from the moment of having clear requirement and access to C5 install or person to do the steps on its side.