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No matter what they say in the end you will see that they are buying oil and gas from Russia by rubble, whether openly or secretly. Because they have no choice but to do so. Europe imports 40% of its oil and gas from Russia. Europe will have to spend a lot of time looking for alternative routes and will have to pay higher prices than could be bought from Russia. I wouldn't call selling oil and gas in rubles a blackmail because if everything can be bought with USD then why not rubble? The USD is a country's currency, the ruble is also a country's currency .
Ruble went down in valuation because of the restrictions imposed upon them and they can demand pay in their currency to balance that and since Russia have the upper hand because majority of the oil is imported to Europe they will be forced to comply as there is no other country that will be able to comply with the demand.