Can I use Sacrifice on a unit that got targeted by a Tactical Retreat earlier and get full value from it?

Yes, you can use Sacrifice on a unit already targeted by Tactical Retreat, and you will receive full value from the spell.

According to RiftJudge FAQ #9425, this interaction allows you to satisfy the cost of Sacrifice while keeping your unit on the board.

Sequence of Events

1. Preparation: A friendly [Mighty] unit is already under the replacement effect of Tactical Retreat (meaning the next time it would die this turn, it will be healed, exhausted, and recalled to your base instead).
2. Playing the Spell: You play Sacrifice and choose that Mighty unit as the target for the additional cost: "kill a friendly [Mighty] unit."
3. Paying Costs: You attempt to pay the cost by killing the unit.
4. Replacement: The replacement effect from Tactical Retreat intercedes. Instead of being sent to the trash, the unit is healed, exhausted, and recalled to your base.
5. Cost Completion: Per Rule 357.2.a, costs replaced by replacement effects are still considered paid. Your cost is successfully paid.
6. Resolution: Sacrifice resolves normally. You draw 2 cards and channel 1 rune.

Summary: You get the full effect of Sacrifice (draw 2 cards, channel 1 rune), and your unit survives the cost payment by being recalled to your base instead of being killed.