If the admin has access to the player-stats edit mode, they can do it while you are offline (and ONLY while you are offline, or it won't stay).
If the admin is me, I do it directly into the player database at SQL level (again, only when you are offline).
If the admin gives it to you in-game, you have to take it to a bank and deposit it, then ideally wait a few moments for it to be synced, and then it's secure. But of course, you are not offlne for this.
Seek clarification from the admin who comped you, as to the method they used.