On the operation type in the inventory you have the reservation method that you can set to manually. 

This is well working with pull rules on routes, when the rule is triggered, on the transfer nothing is reserve

If you create this same operation type and you use it in a push rule 

Units will be reserved even if you choose to reserve them manually 

Solution to Unreserve automatically units on push rule with manual reservation 

If you do this, you will see that once you’ll check availability, as the automated action is on update, it will be automatically unreserved because “check availability” is an update. So to do so add a checkbox and set the domain on this check box also on the automated action. Thanks to that you will be able to not trigger the automated action once the checkbox is checked.