Here's what I did - I added another field to your Units data source called "Saved" - and moved the save logic to it. Clicking the button then sets Saved to a value of 1 - which triggers the logic. I think I left the rule setting the Unit Count on there - you won't need that any more.
So, this means you'll need a helper field of some kind on your secondary data source - if it is a SQL data source, you can use a view to include a blank column. Or if there is another field that you don't need the data for, you can repurpose it - the data remains unchanged at the source, so no need to worry about changing it.