Hey Lars,
The workflow payload and data available on device attributes are separate structures. In order to move a value from the payload onto a device attribute, you’ll need to use a Device State Node to record the data onto the desired device.
Here’s a related forum post with more details. The example in that post is using a workflow to convert Celsius to Fahrenheit.
If you’re triggering the workflow from Device State, using a Device State output will cause the workflow to trigger again. The above link includes instructions for using a conditional node to prevent that loop.