"Assistance" mission allows relocating troops to ally or clan-mate’s city.
In order to launch the mission click on the domain where assistance is needed.

Mission Speed. Assistance" mission speed is twice as fast as attack missions.

Salary.
Troops take Gold needed for their salary for whole duration of the mission, so they don’t consume Gold from the ally or clan-mate’s domain.
The slower travelling speed is, the lower mission’s cost will be.
When launching mission, you specify Assisting troops’ time of staying. The longer is the time, the more Gold will be needed.

Withdrawal.
"Assistance" mission can be withdrawn anytime by initiator or player who is to receive it, even when it’s 1 minute before troops arrive, or when troops are on march, or when troops are already in Assistance Slots.

Rest.
After relocation, troops will not require time for rest.

QUESTION:
There are three players: A, B and C. Player B is an alliance with player A as well as with player C. Now, player A have launched the "Re-seizure" attack against Outer Domain of player C. Player B sent the "Assistance" to player C but troops were late and arrived when the outer domain had been seized by player A. As player B is ally for both players – A and C, will the assistant troops stay in the domain with new owner?
ANSWER:
Yes.