When Russia sells petroleum to Germany, the transaction is completed in USD, ergo both parties need to have USD on-hand in order to trade. This is true for most commodities trade.
if countries suddenly stopped using USD for commodity transactions, then the global demand for the USD will drop.
I don't agree, in most cases the U$Dollars is a standard measure of value rather than physical thingy like a briefcase full of FeRNs (Federal Reserve Notes). Even at that, when the shit hits the fan, money is only worth what you can buy with it and you can't eat gold or burn it to keep warm. If Russia and Germany are buying and selling from each other in a balanced basis, the amounts of currency in the float will be trivial.
Now what upsets the apple cart is when a Russian President like Vladimir Putin, decides he wants Russia playing its old games and prop up al-Assad in Syria, which pisses off the House of Saud who collapses the Oil market and starts to crush the Russian economy. Russia has enough Islamic countries in it sphere of influence (many on its boarders), that keeping them all from turning on either Russia or each other takes a high level of brinkmanship that's difficult to achieve and more difficult to maintain.