Shopify doesn't have an out-of-the-box option to perform this action - I think it would be easier to just not track the inventory in this case.
If you're familiar with building apps though, you could use webhooks and our InventoryLevel API endpoint to update inventory programmatically once it reached a certain level, and I would imagine that one of the inventory management options available in our app store would have functionality that could help you out with this as well.
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