50,000 US servicemen, even with only personal weapons and stiffened with Military Police AFVs are still likely to be a little more than a speed bump to the Germans if they decide on a double cross....
On the other hand, one nuke should just about do the job.
It may be that fuel is handed over in stages. For every "X" tonnes of military vehicles and stores, the Germans hand over "Y" tonnes of oil.
The main point is that there is no way if I were the German command that I'd be simply handing over the oil tanker and hoping the Americans keep their word. May be allies, but it's several years after global nuclear holocaust and my so called allies are (from some points of view) deserting the cause like rats off a sinking ship.