The lawyer clearly loaded the Virginian's saddle bag, with the actual money. You can see this easily, then see that his money box was emptied, and that he did not leave the area where he was. Any unusual trickery, would have been able to be seen by the Virginian.
When the deputy lets Swenson out of his jail cell, he turns the key in the lock. When the door opens, there is no locking device on the side of the cell door.