Hi,
At the moment, changing stock quantity on a product isn’t supported directly via any existing action. We are planning to introduce product triggers, filters and actions in version 1.6.0 sometime early 20201.
You could create a “Change Custom Field” action that updates the post meta field “_stock” on a Post object. But that would take some tinkering without any guarantee that it will solve your use case.
If you want further help with your specific case, please get in contact with us via our support pages: https://www.wundermatics.com/support/contact/