Black Hills Ambush (1952)

"DANGER ON THE BLACK HILLS TRAIL! Two-gun"

U.S. Marshal Rocky Lane arrives to help his friend Nugget whose shipments are being robbed. Learning the gang only goes for incoming shipments, he figures they are repacking the loot and having Nugget ship it out. So he hijacks the next outgoing shipment finding a box labeled tools contains gold bars. Then posing as an outlaw he makes a deal with the gang hoping to round them up. But the boss learns he is a Marshal and he is made a prisoner.