When a minion is sent back to your barracks (perhaps by an enemy talent or relic), what happens to it depends on what kind of minion it is.

If it is a minion native to that faction, it can be deployed again on a future wave but the CP cost will need to be paid again. 

If it is a merc minion (or in rare instances, a converted landmark minion), it is placed beside your barracks as normal for minions not native to the faction, and can be redeployed on a future wave without a CP cost. 

Note that if you are playing as The Uprising, you will still need to spend CP cost equal to the Source Cost to deploy a merc minion (though you would still be able to deploy converted landmark minions for free).