Within the first 50 seconds after you send your units outside of the Base ( Raid / Occupation / etc.), you may cancel the action and recall the units for free.
However, in order to recall the units after 50 seconds have passed, you will need to use the "Instant Callback" item.
You may use it in the Tracking tab of your Command HQ.
If you try to recall the units without having the item, you will receive the following notification:
You will be charged 300 Diamonds in case you press "Yes".
Once you do so, the "Army Callback" item will be automatically purchased and applied to recall the troops you intended to recall.